Yes ok he no longer reincarnates but what happens to him?

Nibbana is ego death, the end of "I am." There is no him for something to happen to; no framework of self to hang experience. All experience is explained subjectively, and the subject has been removed.

Then what should I care about my future life?
...
So what should I care about accumulating merits or demerits in the future life? It will not be me who lives it!

The same reason we wipe our asses, so we don't leave a mess. Don't let ultimate reality cloud conventional judgement.

There is no accumulation of merits; one accumulates and untrains habits of body, speech, and mind.

Having no regard for one's intentional actions (kamma) and their results (vipaka) is like an alcoholic not caring about the liver of his future self because it's not his liver. It may be ultimately true, but it's conventionally stupid and leads to obvious affliction.

The illusion is that on a foundational level, we believe things to be permanent, stable, certain, unchanging, reliable, and as belonging to us, "I, me, or mine." In reality, everything we experience is Dependently Originated, impermanent, unstable, uncertain, changing, unreliable, and chaotic like a dream. “Whatever is subject to origination is all subject to cessation.” (SN 56.11)

My idea was to explain that in dreams, there are real psychological elements that show truths in our world, but it sounds like you're suggesting that even these things too, are not necessarily truths.

To be awakened is to be awake to reality, the way things are, Dhamma. One could reflect on dreams; investigate the truth of their nature, see that they are inherently impermanent, unsatisfactory, and not-self. All conditioned phenomena, physical and mental, have those three characteristics and can be investigated to cultivate wisdom. You may just have to take a different angle.

The Buddha didn't suggest that life is a dream in the Suttas, but in the Diamond Sutra, the impermanent, transient nature of conditioned phenomena is compared to a dream.

So I say to you – This is how to contemplate our conditioned existence in this fleeting world:
Like a tiny drop of dew, or a bubble floating in a stream;
Like a flash of lightning in a summer cloud,
Or a flickering lamp, an illusion, a phantom, or a dream.
So is all conditioned existence to be seen.

All of the constituent parts of a being, that make up our conditioned existence, are found in the five aggregates of form, feeling, perception, volition, and consciousness. They are impermanent, not under total control, and as a result are suffering. What is suffering is not-self (SN 22.15). There is no permanent self, soul, essence, consciousness, etc. to be found anywhere, even Nibbana is without a permanent self (MN 1).

You should truly see any kind of feeling … perception … choices … consciousness at all—past, future, or present; internal or external; solid or subtle; inferior or superior; far or near: all consciousness—with right understanding: ‘This is not mine, I am not this, this is not my self.’ - SN 22.49: Soṇasutta

MN 26: Pāsarāsisutta tells of the Buddhas enlightenment.

“Then, bhikkhus, being myself subject to birth, having understood the danger in what is subject to birth, seeking the unborn supreme security from bondage, Nibbāna, I attained the unborn supreme security from bondage, Nibbāna; being myself subject to ageing, having understood the danger in what is subject to ageing, seeking the unageing supreme security from bondage, Nibbāna, I attained the unageing supreme security from bondage, Nibbāna; being myself subject to sickness, having understood the danger in what is subject to sickness, seeking the unailing supreme security from bondage, Nibbāna, I attained the unailing supreme security from bondage, Nibbāna; being myself subject to death, having understood the danger in what is subject to death, seeking the deathless supreme security from bondage, Nibbāna, I attained the deathless supreme security from bondage, Nibbāna; being myself subject to sorrow, having understood the danger in what is subject to sorrow, seeking the sorrowless supreme security from bondage, Nibbāna, I attained the sorrowless supreme security from bondage, Nibbāna; being myself subject to defilement, having understood the danger in what is subject to defilement, seeking the undefiled supreme security from bondage, Nibbāna, I attained the undefiled supreme security from bondage, Nibbāna. The knowledge and vision arose in me: ‘My deliverance is unshakeable; this is my last birth; now there is no renewal of being.’"

Nibbana is not a place or afterlife and is without dimension or consciousness. It is the extinguishment of ignorance with greed, hatred, and delusion; of "I am."

Based on this particular Sutta,

They rely on these five powers of a trainee: faith, conscience, prudence, energy, and wisdom. And these five faculties manifest in them strongly: faith, energy, mindfulness, immersion, and wisdom.

With faith, one takes on the training.
Faith turns into conviction having verified the training for oneself.

Cultivating energy:

1. don’t focus on or cultivate the perception that you were meditating on when you fell drowsy
2. Then think about and consider the teaching as you’ve learned and memorized it, examining it with your mind
3. Then rehearse in detail the teaching as you’ve learned and memorized it.
4. Then pinch your ears and rub your limbs.
5. Then get up from your seat, flush your eyes with water, look around in every direction, and look up at the stars and constellations.
6. Then apply your mind to the perception of light, focusing on the perception of day: as by day, so by night; as by night, so by day. And so, with an open and unenveloped heart, develop a mind that’s full of radiance.
7. Then walk mindfully, focusing on the perception of before and behind, your faculties directed inwards and your mind not scattered outside.
8. Then lie down in the lion’s posture—on the right side, placing one foot on top of the other—mindful and aware, and focused on the time of getting up. When you wake, you should get up quickly, thinking: ‘I will not live indulging in the pleasures of sleeping, lying down, and drowsing.’

Cultivating The Four Foundations of Mindfulness: DN 22: Mahāsatipaṭṭhānasutta

Immersion, samma samadhi, is conditioned by consciousness and prudence, sense restraint, and ethical conduct. "when there is no conscience and prudence [moral shame and fear of wrongdoing], one who lacks conscience and prudence has destroyed a vital condition for sense restraint. When there is no sense restraint, one who lacks sense restraint has destroyed a vital condition for ethical conduct. When there is no ethical conduct, one who lacks ethics has destroyed a vital condition for right immersion." It is a mind without the five hinderances.

And wisdom, the enemy of ignorance, is cultivated through investigating Dhamma while in Samma Samadhi.

When their mind has become immersed in samādhi like this—purified, bright, flawless, rid of corruptions, pliable, workable, steady, and imperturbable—they extend it toward knowledge of the ending of defilements. They truly understand: ‘This is suffering’ … ‘This is the origin of suffering’ … ‘This is the cessation of suffering’ … ‘This is the practice that leads to the cessation of suffering.’ They truly understand: ‘These are defilements’ … ‘This is the origin of defilements’ … ‘This is the cessation of defilements’ … ‘This is the practice that leads to the cessation of defilements.’ Knowing and seeing like this, their mind is freed from the defilements of sensuality, desire to be reborn, and ignorance. - MN 39: Mahāassapurasutta

There are two types of hope:

1. Hope rooted in wrong intention.
2. Hope rooted in right intention.

Intention

“Therein, bhikkhus, right view comes first. And how does right view come first? One understands wrong intention as wrong intention and right intention as right intention: this is one’s right view.

“And what, bhikkhus, is wrong intention? The intention of sensual desire, the intention of ill will, and the intention of cruelty: this is wrong intention.

“And what, bhikkhus, is right intention? Right intention, I say, is twofold: there is right intention that is affected by taints, partaking of merit, ripening in the acquisitions, and there is right intention that is noble, taintless, supramundane, a factor of the path.

“And what, bhikkhus, is right intention that is affected by taints, partaking of merit, ripening in the acquisitions? The intention of renunciation, the intention of non-ill will, and the intention of non-cruelty: this is right intention that is affected by taints…ripening in the acquisitions.

“And what, bhikkhus, is right intention that is noble, taintless, supramundane, a factor of the path? The thinking, thought, intention, mental absorption, mental fixity, directing of mind, verbal formation in one whose mind is noble, whose mind is taintless, who possesses the noble path and is developing the noble path: this is right intention that is noble…a factor of the path.

“One makes an effort to abandon wrong intention and to enter upon right intention: this is one’s right effort. Mindfully one abandons wrong intention, mindfully one enters upon and abides in right intention: this is one’s right mindfulness. Thus these three states run and circle around right intention, that is, right view, right effort, and right mindfulness." - MN 117: Mahācattārīsakasutta

You are correct in that the creation of bright kamma with bright results is not the end goal.
Nibbana is the stilling of all formations, the ending of all deeds.

However, it's important to remember that the training is gradual.
There is:

1. Painful practice with slow insight,
2. painful practice with swift insight,
3. pleasant practice with slow insight, and
4. pleasant practice with swift insight. - AN 4.163: Asubhasutta

Most of us will have to slowly tame and develop the mind by intentionally training on The Noble Eightfold Path.

From one angle, the training begins with moral shame and fear of wrongdoing, hirī ottappa.

Mendicants, when there is no conscience and prudence [moral shame and fear of wrongdoing], one who lacks conscience and prudence has destroyed a vital condition for sense restraint. When there is no sense restraint, one who lacks sense restraint has destroyed a vital condition for ethical conduct. When there is no ethical conduct, one who lacks ethics has destroyed a vital condition for right immersion. When there is no right immersion, one who lacks right immersion has destroyed a vital condition for true knowledge and vision. When there is no true knowledge and vision, one who lacks true knowledge and vision has destroyed a vital condition for disillusionment and dispassion. When there is no disillusionment and dispassion, one who lacks disillusionment and dispassion has destroyed a vital condition for knowledge and vision of freedom.

Part of Right View is understanding that there is fruit to good and bad actions. Virtuous behavior cultivates non-regret, which gradually leads to the direct knowledge and vision of liberation, Nibbana.

Thus, bhikkhus, (9)–(10) the knowledge and vision of liberation is the purpose and benefit of disenchantment and dispassion; (8) disenchantment and dispassion are the purpose and benefit of the knowledge and vision of things as they really are; (7) the knowledge and vision of things as they really are is the purpose and benefit of concentration; (6) concentration is the purpose and benefit of pleasure; (5) pleasure is the purpose and benefit of tranquility; (4) tranquility is the purpose and benefit of rapture; (3) rapture is the purpose and benefit of joy; (2) joy is the purpose and benefit of non-regret; and (1) non-regret is the purpose and benefit of virtuous behavior. - AN 10.2: Cetanākaraṇīyasutta

A wonderful and short book on the practice can be found at: The Noble Eightfold Path: The Way to the End of Suffering by Bhikkhu Bodhi.

The moment-to-moment training is laid out in Suttas such as MN 27, MN 39, MN 107.

• Ethical conduct (sila/the five precepts)
• Sense restraint (sister to sila)
• Eating in moderation
• Committing to wakefulness
• Mindfulness and situational awareness

These practices condition and prepare the mind for meditation, and seclusion from the five hinderances, Samma Samadhi, Right Concentration. This is why The Noble Eightfold Path is summarized as sila, samadhi, panna (wisdom). Ethics conditions the mind for meditation, samadhi, and samadhi is then a tool to cultivate wisdom through investigation. Wisdom reinforces ethical conduct and sense restraint, creating a feedback loop.

Here are short talks on the gradual training by Ajahn Kovilo and Ajahn Nisabho:
MN 27: Cūḷahatthipadopama Sutta - The Elephant’s Footprint & Gradual Training | 10-Minute Majjhima
MN 39: Mahā Assapura Sutta - The True Recluse, Gradual Training & Jhāna Similes | 10-Minute Majjhima
MN 107: Gaṇakamoggallāna Sutta - Step by Step to Freedom | 10-Minute Majjhima

Regardless of what tradition you choose, The Four Noble Truths are the core teaching, and the Fourth Truth is The Noble Eightfold Path, the gradual training.

Can Buddhism influence a person to be asexual?

Yes. Monastics, and lay practitioners that adhere to the 8 precepts, practice celibacy. They meditate on the repulsive features of the body to intentionally diminish and eliminate sexual desires.

Could Buddhism have influenced this?

If you have been intentionally cultivating wisdom through mindfulness of the body, then possibly. Renunciations, such as celibacy, naturally occur as wisdom grows. One becomes disenchanted and dispassionate toward sights, sounds, smells, tastes, touches, and ideas; toward the body, mind, and all things impermanent, suffering, and not-self (MN 147).

How does Buddhism reconcile historical and ongoing wars, violence, invasions, destruction and conquest?

They are to be expected.

All conditioned phenomena are marked with impermanenece and suffering.
What is peaceful will become otherwise. What is violent will become otherwise.

All beings are Dependently Originated with ignorance as condition for craving.
Craving, the origin of suffering, then leads to wars, violence, invasions, destruction and conquest:

Craving gives rise to searching. Searching gives rise to gaining material things. Gaining material things gives rise to evaluation. Evaluation gives rise to desire and lust. Desire and lust gives rise to attachment. Attachment gives rise to ownership. Ownership gives rise to stinginess. Stinginess gives rise to safeguarding. Owing to safeguarding, many bad, unskillful things come to be: taking up the rod and the sword, quarrels, arguments, and disputes, accusations, backbiting, and lies. These are the nine things rooted in craving. - AN 9.23: Taṇhāmūlakasutta

The Middle Way is putting down all pursuit of sensual pleasure, and self-mortification. Neither lead to liberation. This includes travel for the sake of sights, sounds, smells, tastes, touches, and ideas, eating for certain tastes, etc.,

Mendicants, these two extremes should not be cultivated by one gone forth. What two? Indulgence in sensual pleasures, which is low, crude, ordinary, ignoble, and pointless. And indulgence in self-mortification, which is painful, ignoble, and pointless.

But it's important to remember that The Noble Eightfold Path is a gradual training. Renunciation occurs naturally, the pursuit of sensual pleasures falls away, as wisdom is cultivated. With the cultivation of wisdom, craving for sensual pleasures, craving existence, and craving non-existence are abandoned.

One becomes disenchanted with sights, sounds, smells, tastes, touches, ideas, the eyes, ears, nose, tongue, body, and mind, corresponding contacts, feelings, perceptions, and consciousness that arise from causes and condition, without agency or control - self.

“Seeing thus, Rāhula, a well-taught noble disciple becomes disenchanted with the eye, disenchanted with forms, disenchanted with eye-consciousness, disenchanted with eye-contact, and disenchanted with anything comprised within the feeling, perception, formations, and consciousness that arise with eye-contact as condition.

“He becomes disenchanted with the ear…He becomes disenchanted with the nose…He becomes disenchanted with the tongue…He becomes disenchanted with the body…He becomes disenchanted with the mind, disenchanted with mind-objects, disenchanted with mind-consciousness, disenchanted with mind-contact, and disenchanted with anything comprised within the feeling, perception, formations, and consciousness that arise with mind-contact as condition.

“Being disenchanted, he becomes dispassionate. Through dispassion his mind is liberated. When it is liberated there comes the knowledge: ‘It is liberated.’ He understands: ‘Birth is destroyed, the holy life has been lived, what had to be done has been done, there is no more coming to any state of being.’” - MN 147: Cūḷarāhulovādasutta

Aspirations toward the cessation of suffering are beneficial, but aspirations toward sensual pleasures, or of being, are not.

Everyone has an origin story.

Choose, intentionally, not to pick up the narrative. It doesn't belong to you; nothing truly does.
"You" are not thoughts, feelings, trauma, or narrative.

When you see you own nothing, nothing can harm.
Even suffering cannot be owned and does not belong to anyone. Suffering is not-self.

Intentionally develop the perceptions of impermanence and not-self.

See that all the parts and processes that make up a person are transient and not truly "I, me, or mine."
They come into being through causes and conditions beyond our control and are unsatisfactory, dukkha.

when the perception of impermanence is developed and cultivated it eliminates all desire for sensual pleasures, for rebirth in the realm of luminous form, and for rebirth in a future life. It eliminates all ignorance and eradicates all conceit ‘I am’. - SN 22.102: Aniccasaññāsutta

If the thoughts were true, and you, would you have chosen those thoughts? No. So they cannot be yours. No choice, no control, not-self. Not yours, there's no stable, permanent person to be a bad person, or superior, or equal.

“Soṇa, there are ascetics and brahmins who—based on form, which is impermanent, suffering, and perishable—regard themselves thus: ‘I’m better’, or ‘I’m equal’, or ‘I’m worse’. What is that but a failure to see truly? Based on feeling … perception … choices … consciousness, which is impermanent, suffering, and perishable, they regard themselves thus: ‘I’m better’, or ‘I’m equal’, or ‘I’m worse’. What is that but a failure to see truly?

When we claim the body, feelings, perceptions, volitions, and consciousness as I, me, or mine, we pickup a heavy burden.

“The five aggregates are truly burdens,
The burden-carrier is the person.
Taking up the burden is suffering in the world,
Laying the burden down is blissful." - SN 22.22: Bhārasutta

Investigate the nature of thoughts, and the five aggregates.
Investigate how your thoughts come and go, arise and cease.

When there is sensory contact with sights, sounds, smells, tastes, touchs, and thoughts, there are feelings and perceptions. What you perceive, you think about.

Contact is a requirement for feeling. What you feel, you perceive. What you perceive, you think about. What you think about, you proliferate. What you proliferate is the source from which judgments driven by proliferating perceptions beset a person. This occurs with respect to sights known by the eye in the past, future, and present.

We are not shallow, or happy, or angry, or good, or bad. Sometimes there are shallow thoughts. Sometimes there is happiness. Sometimes there is anger. Sometimes there are good intentional actions and bad intentional actions, volitions. All arise based on causes and conditions such as ignorance, the six sense bases, contact, feeling, perception, and consciousness.

You should truly see any kind of feeling … perception … choices … consciousness at all—past, future, or present; internal or external; solid or subtle; inferior or superior; far or near: all consciousness—with right understanding: ‘This is not mine, I am not this, this is not my self.’ - SN 22.49: Soṇasutta

The Noble Eightfold Path: The Way to the End of Suffering - what is needed.

I don't believe 4th Jhana is required. There are Suttas, like MN 147: Cūḷarāhulovādasutta, where full enlightenment occurs during active dialogue and listening:

“Is what is impermanent, suffering, and subject to change fit to be regarded thus: ‘This is mine, this I am, this is my self’?”—“No, venerable sir.”

“Seeing thus, Rāhula, a well-taught noble disciple becomes disenchanted...

That is what the Blessed One said. The venerable Rāhula was satisfied and delighted in the Blessed One’s words. Now while this discourse was being spoken, through not clinging the venerable Rāhula’s mind was liberated from the taints.

All parts that construct a being are found in the five aggregates of form, feelings, perceptions, volitions, and consciousness. There is nothing outside the five aggregates. All constituent parts and processes are impermanent, dukkha, and not-self (SN 22.59).

From the Buddhist perspective, a spirit is a type of being with five aggregates, not a permanent part of the five aggregates that continues. An example would be a peta (a hungry ghost) or a yakkha. One is reborn as such a being depending on one's intentional actions, kamma. I have never heard of "personal" spirits, but there may be other invisible beings around you.

THE BUDDHIST COSMOS by Ajahn Punnadhammo is a free and wonderful resource on the Buddhist cosmology, including all the realms and various types of beings such as tree spirits or devas and hungry ghosts.

How should I go about this?

If you're interested in Buddhism, the Buddha taught every being be treated with unconditional, limitless goodwill and friendliness (metta), and to practice The Noble Eightfold Path: The Way to the End of Suffering.

Not-self is a perception to be developed.

“Mendicants, I will teach you seven principles that prevent decline. Listen and apply your mind well … And what are the seven principles that prevent decline? As long as the mendicants develop the perception of impermanence … not-self … ugliness … drawbacks … giving up … fading away … cessation, they can expect growth, not decline. As long as these seven principles that prevent decline last among the mendicants, and as long as the mendicants are seen following them, they can expect growth, not decline.”

And a truth about the way things are, Dhamma. Seeing the truth, and directly knowing, leads to liberation:

“Is what is impermanent, suffering, and subject to change fit to be regarded thus: ‘This is mine, this I am, this is my self’?”—“No, venerable sir.”

“Seeing thus, Rāhula, a well-taught noble disciple becomes disenchanted with the eye, disenchanted with forms, disenchanted with eye-consciousness, disenchanted with eye-contact, and disenchanted with anything comprised within the feeling, perception, formations, and consciousness that arise with eye-contact as condition.

“He becomes disenchanted with the ear…He becomes disenchanted with the nose…He becomes disenchanted with the tongue…He becomes disenchanted with the body…He becomes disenchanted with the mind, disenchanted with mind-objects, disenchanted with mind-consciousness, disenchanted with mind-contact, and disenchanted with anything comprised within the feeling, perception, formations, and consciousness that arise with mind-contact as condition.

“Being disenchanted, he becomes dispassionate. Through dispassion his mind is liberated. When it is liberated there comes the knowledge: ‘It is liberated.’ He understands: ‘Birth is destroyed, the holy life has been lived, what had to be done has been done, there is no more coming to any state of being.’” - MN 147: Cūḷarāhulovādasutta

How do you go about it?

Practice The Noble Eightfold Path, the gradual training. Practice ethical conduct, sense restraint, eating in moderation, be committed to wakefulness, mindfulness, and situational awareness to condition the mind for meditation. The mind must be secluded from the five hinderances for Samma Samadhi, Right Concentration.

Secluded from the hindrances, observe impermanence, fading away, cessation, and letting go; The Four Noble Truths in the meditation object.

Seeing Dependent Originationand Not-self, one sees through the five aggregates of form, feelings, perceptions, volitions, and consciousness, and puts down the burden of ownership, control, and responsibility for what is not ownable, controllable, or "I, me, mine."

“The five aggregates are indeed burdens,
and the individual is the bearer of the burden.
Taking up the burden is suffering in the world,
and putting the burden down is happiness.

When the heavy burden is put down
without taking up another,
one who has plucked out craving, root and all,
is hungerless, quenched.” - SN 22.22: Bhārasutta

This is the perception of not-self and how it is developed.
Once developed, I-making and mine-making diminish, and cease at Arahantship.

Meditation and mindfulness are already accepted and practiced in modern psychology.

• What is Cognitive Behavioral Therapy?
• Mindfulness-Based Cognitive Therapy | Psychology Today
• MBSR Program | CFM | UMass Memorial Health

These help a patient retrain behavioral patterns; how they respond to stimuli.

Psychology and psychiatry are interested in establishing a "healthy" self-image, identity, level of self-esteem, and providing the skills necessary to at least live with a bearable amount of suffering.

The Noble Eightfold Path also retrains behavioral patterns of body, speech, and mind, but is interested in the complete cessation of suffering, which includes the realization of not-self, the ending of "I-making" and "mine-making."

Due to the brain's neuroplasticity, the training can rewire the structures of the brain, changing one's perceptions of reality and how they interact with the world, but it does not directly correct hormonal and chemical imbalances.

Medication should be taken when medication is required whether practicing The Noble Eightfold Path or not.

Letting go of emotions, or anything, is a side effect of wisdom. Wisdom is cultuvated through investigation.

Investigate how these emotions arose. You will find that they were conditioned by sense contact and shaped by perception (MN 18). You wont find control, a self summoning the emotions. When observed and known directly that they arise based on causes and conditions beyond control, there will be letting go of the burden.

The Noble Eightfold Path is a gradual training, and we all have different starting conditions.

The moment-to-moment practices are outlined in Suttas such as MN 27, MN 39, MN 107.

• Ethical conduct (sila/the five precepts)
• Sense restraint (sister to sila)
• Eating in moderation
• Committing to wakefulness
• Mindfulness and situational awareness

These practices condition and prepare the mind for meditation, and seclusion from the five hinderances, Samma Samadhi, Right Concentration. This is why The Noble Eightfold Path is summarized as sila, samadhi, panna (wisdom). Ethics conditions the mind for meditation, samadhi, and samadhi is then a tool to cultivate wisdom through investigation. Wisdom reinforces ethical conduct and sense restraint, creating a feedback loop.

“Mendicants, when there is no conscience and prudence, one who lacks conscience and prudence has destroyed a vital condition for sense restraint. When there is no sense restraint, one who lacks sense restraint has destroyed a vital condition for ethical conduct. When there is no ethical conduct, one who lacks ethics has destroyed a vital condition for right immersion [Samma Samadhi]. When there is no right immersion, one who lacks right immersion has destroyed a vital condition for true knowledge and vision. When there is no true knowledge and vision, one who lacks true knowledge and vision has destroyed a vital condition for disillusionment and dispassion. When there is no disillusionment and dispassion, one who lacks disillusionment and dispassion has destroyed a vital condition for knowledge and vision of freedom. [Nibbana]" - AN 7.65: Hirīottappasutta

Meditation is conditioned by other practices such as ethical conduct, sense restraint, mindfulness with situational awareness, and renunciation (MN 39). Without putting in the causes and conditions, preparing the mind, there will be little fruit.

It's easiest to establish a habit of meditation when The Noble Eightfold Path is one's way of life. Meditation, or Right Concentration, is supported by the seven other path factors, the culmination of training.

Success can take minutes or years depending on an individual's conditioning and where they are on the gradual training.

Effects come with the cultivation of wisdom.

Calming the mind is only one part. Once the mind is settled and serene, one investigates the nature of reality. With investigation, insights arise. With insights, there is wisdom; wisdom diminishes and destroys ignorance. With the diminishing and destruction of ignorance, one experiences results.

Here are the instructions provided by the Buddha to his son, Ven. Rahula:

Develop mindfulness of breathing. When mindfulness of breathing is developed and cultivated it’s very fruitful and beneficial. And how is mindfulness of breathing developed and cultivated to be very fruitful and beneficial?

It’s when a mendicant—gone to a wilderness, or to the root of a tree, or to an empty hut—sits down cross-legged, sets their body straight, and brings mindfulness to the present. Just mindful, they breathe in. Mindful, they breathe out.

Breathing in heavily they know: ‘I’m breathing in heavily.’ Breathing out heavily they know: ‘I’m breathing out heavily.’ Breathing in lightly they know: ‘I’m breathing in lightly.’ Breathing out lightly they know: ‘I’m breathing out lightly.’ They practice like this: ‘I’ll breathe in experiencing the whole body.’ They practice like this: ‘I’ll breathe out experiencing the whole body.’ They practice like this: ‘I’ll breathe in stilling the physical process.’ They practice like this: ‘I’ll breathe out stilling the physical process.’

They practice like this: ‘I’ll breathe in experiencing rapture.’ They practice like this: ‘I’ll breathe out experiencing rapture.’ They practice like this: ‘I’ll breathe in experiencing bliss.’ They practice like this: ‘I’ll breathe out experiencing bliss.’ They practice like this: ‘I’ll breathe in experiencing the mental process.’ They practice like this: ‘I’ll breathe out experiencing the mental process.’ They practice like this: ‘I’ll breathe in stilling the mental process.’ They practice like this: ‘I’ll breathe out stilling the mental process.’

They practice like this: ‘I’ll breathe in experiencing the mind.’ They practice like this: ‘I’ll breathe out experiencing the mind.’ They practice like this: ‘I’ll breathe in gladdening the mind.’ They practice like this: ‘I’ll breathe out gladdening the mind.’ They practice like this: ‘I’ll breathe in immersing the mind in samādhi.’ They practice like this: ‘I’ll breathe out immersing the mind in samādhi.’ They practice like this: ‘I’ll breathe in freeing the mind.’ They practice like this: ‘I’ll breathe out freeing the mind.’

They practice like this: ‘I’ll breathe in observing impermanence.’ They practice like this: ‘I’ll breathe out observing impermanence.’ They practice like this: ‘I’ll breathe in observing fading away.’ They practice like this: ‘I’ll breathe out observing fading away.’ They practice like this: ‘I’ll breathe in observing cessation.’ They practice like this: ‘I’ll breathe out observing cessation.’ They practice like this: ‘I’ll breathe in observing letting go.’ They practice like this: ‘I’ll breathe out observing letting go.’

Mindfulness of breathing, when developed and cultivated in this way, is very fruitful and beneficial. When mindfulness of breathing is developed and cultivated in this way, even as the final breaths cease they are known, not unknown.” - MN 62: Mahārāhulovādasutta

A mind immersed in Samma Samadhi is a mind is free, secluded from of the five hinderances of sensual desire, ill will, sloth and torpor, restlessness and remorse, and doubt.

Secluded from the hindrances, observe impermanence, fading away, cessation, and letting go; The Four Noble Truths in the breath.

To know the breath as Dependently Originated and Not-self, one sees through the five aggregates of form, feelings, perceptions, volitions, and consciousness, and puts down the burden of ownership, control, and responsibility for what is not ownable, controllable, or "I, me, mine."

“The five aggregates are indeed burdens,
and the individual is the bearer of the burden.
Taking up the burden is suffering in the world,
and putting the burden down is happiness.

When the heavy burden is put down
without taking up another,
one who has plucked out craving, root and all,
is hungerless, quenched.” - SN 22.22: Bhārasutta