What is happening to Lord Buddha after "parinirvana"?

People should stop this type of questions. I am no expert but unless you experience the thing they call nirvana or anything close to that, you'll never understand anything about that. People here can give you all their interpretations but that would only they can give you that much.

This is clearly answered by Itivuttaka 44 with excellent commentary by Thanissaro Bhikkhu:

This was said by the Blessed One, said by the Arahant, so I have heard: "Monks, there are these two forms of the Unbinding property. Which two? The Unbinding property with fuel remaining, & the Unbinding property with no fuel remaining.

And what is the Unbinding property with fuel remaining? There is the case where a monk is an arahant whose fermentations have ended, who has reached fulfillment, finished the task, laid down the burden, attained the true goal, ended the fetter of becoming, and is released through right gnosis. His five sense faculties still remain and, owing to their being intact, he is cognizant of the agreeable & the disagreeable, and is sensitive to pleasure & pain. His ending of passion, aversion, & delusion is termed the Unbinding property with fuel remaining. (see Note1)

And what is the Unbinding property with no fuel remaining? There is the case where a monk is an arahant whose fermentations have ended, who has reached fulfillment, finished the task, laid down the burden, attained the true goal, ended the fetter of becoming, and is released through right gnosis. For him, all that is sensed, being unrelished, will grow cold right here. This is termed the Unbinding property with no fuel remaining." (see Note2)

Commentary (Note1 and Note2) by Thanissaro Bhikkhu:

With fuel remaining (sa-upadisesa) and with no fuel remaining (anupadisesa): The analogy here is to a fire. In the first case, the flames are out, but the embers are still glowing. In the second, the fire is so thoroughly out that the embers have grown cold. The "fuel" here is the five aggregates. While the arahant is still alive, he/she still experiences the five aggregates, but they do not burn with the fires of passion, aversion, or delusion. When the arahant passes away, there is no longer any experience of aggregates here or anywhere else. For a discussion of this point, see The Mind Like Fire Unbound, pp. 21-37.

No detailed description is available for one's state after parinirvana. However, please note that budhha who had attained all the Jhanas preferred parinirvana.

Realm of neither perception nor no perception (last Jhanas) can be described as state of permanent ecstasy. Even for this state it is hard to find detailed description.

We can imagine that parinirvana will be beyond description.

Nothing. That's THE END ! His conscience is gone, and there's nothing left of him. That's the meaning of parinibbana.

examine the nature of the question;

"What happens?" It is really difficult to answer a "what happens?" even when one talks about the nature of things as we know it.

If you see an apple then seeing happens. What is the seeing then exactly? Whatever we think seeing ultimately is, well it isn't that because seeing is not thinking. Nor can it be explained in terms of other sensory perception and however it is expressed it is only an expression.

As a matter of fact we can't really talk about a reality beyond the six classes of perception because we don't understand reality in terms of anything other than those very classes of perception.

If this is understood then one can understand that whatever it is that appears to happen in this life is an expression of causes and conditions for it's appearance.

Parinibbana disignates the complete extinguishing of causes and conditions for the appearance of the various classes of perception which make up our reality.

That is as far as we can go in this reality and beyond this point our reality ends. What is not included in the Allness of the All can not be explained in terms of something included in the Allness of the All.

[Sariputta:] "The statement, 'With the remainderless stopping & fading of the six contact-media [vision, hearing, smell, taste, touch, & intellection] is it the case that there is anything else?' objectifies non-objectification.[1] The statement, '... is it the case that there is not anything else ... is it the case that there both is & is not anything else ... is it the case that there neither is nor is not anything else?' objectifies non-objectification. However far the six contact-media go, that is how far objectification goes. However far objectification goes, that is how far the six contact media go. With the remainderless fading & stopping of the six contact-media, there comes to be the stopping, the allaying of objectification. https://www.accesstoinsight.org/tipitaka/an/an04/an04.174.than.html

In other words one can't really pin down the existence of a being even in this life;

"What do you think, Anuradha: Is form constant or inconstant?"

"Inconstant, lord."

"And is that which is inconstant easeful or stressful?"

"Stressful, lord."

"And is it proper to regard what is inconstant, stressful, subject to change as: 'This is mine. This is my self. This is what I am'?"

"No, lord."

"Is feeling constant or inconstant?"

"Inconstant, lord."...

"Is perception constant or inconstant?"

"Inconstant, lord."...

"Are fabrications constant or inconstant?"

"Inconstant, lord."...

"Is consciousness constant or inconstant?

"Inconstant, lord."

"And is that which is inconstant easeful or stressful?"

"Stressful, lord."

"And is it proper to regard what is inconstant, stressful, subject to change as: 'This is mine. This is my self. This is what I am'?"

"No, lord."

"What do you think, Anuradha: Do you regard form as the Tathagata?"

"No, lord."

"Do you regard feeling as the Tathagata?"

"No, lord."

"Do you regard perception as the Tathagata?"

"No, lord."

"Do you regard fabrications as the Tathagata?"

"No, lord."

"Do you regard consciousness as the Tathagata?"

"No, lord."

"What do you think, Anuradha: Do you regard the Tathagata as being in form?... Elsewhere than form?... In feeling?... Elsewhere than feeling?... In perception?... Elsewhere than perception?... In fabrications?... Elsewhere than fabrications?... In consciousness?... Elsewhere than consciousness?"

"No, lord."

"What do you think: Do you regard the Tathagata as form-feeling-perception-fabrications-consciousness?"

"No, lord."

"Do you regard the Tathagata as that which is without form, without feeling, without perception, without fabrications, without consciousness?"

"No, lord."

"And so, Anuradha — when you can't pin down the Tathagata as a truth or reality even in the present life — is it proper for you to declare, 'Friends, the Tathagata — the supreme man, the superlative man, attainer of the superlative attainment — being described, is described otherwise than with these four positions: The Tathagata exists after death, does not exist after death, both does & does not exist after death, neither exists nor does not exist after death'?"

"No, lord."

"Very good, Anuradha. Very good. Both formerly & now, it is only stress that I describe, and the cessation of stress."

Enlightenment (Bodhi) is awakening to a different perspective, in which everything is interconnected and alive, constantly flowing and transforming but never getting born and never dying.

Having directly known the extent of designation and the extent of the objects of designation, the extent of expression and the extent of the objects of expression, the extent of description and the extent of the objects of description, the extent of discernment and the extent of the objects of discernment, the extent to which the cycle revolves: Having directly known that, the monk is released.

But, Master Gotama, the monk whose mind is thus released: Where does he reappear?

"Reappear,' Vaccha, doesn't apply.

In that case, Master Gotama, he does not reappear.

"Does not reappear,' Vaccha, doesn't apply.

...

Of course you're befuddled, Vaccha. Of course you're confused. Deep, Vaccha, is this Dharma, hard to see, hard to realize, tranquil, refined, beyond the scope of conjecture, subtle, to-be-experienced by the wise.

...

Any identity by which one describing the Tathagata would describe him: That the Tathagata has abandoned ... Freed from identity, Vaccha, the Tathagata is deep, boundless, hard to fathom, like the sea. 'Reappears' doesn't apply. 'Does not reappear' doesn't apply.

The question is based on an invalid framework! There was no 'him' in the first place. When the cause of the future arising of the 5 aggregates has been eliminated, there will be no more arising.

I am not as knowledgeable in these matters as many others I see in this Exchange, but it seems to me that the answer to this question might fall under the categories of the 14 Unanswered Questions #'s 11-14 - Pali Canon or the 16 Unwise Reflections - Sabbasava Sutta. (Although the latter seems to focus on self reflection and, so, may not apply to reflections on others).

If that is the case, the question may not have an answer from the Buddha.

I'd appreciate correction if my answer is without merit.

It is something undeclared by The Buddha.

These four views are undeclared by The Buddha:

• After death a Tathāgata exists
• After death a Tathāgata does not exist
• After death a Tathāgata both exists & does not exist
• After death a Tathāgata neither exists nor does not exist

"Any consciousness by which one describing the Tathagata would describe him: That the Tathagata has abandoned, its root destroyed, made like a palmyra stump, deprived of the conditions of development, not destined for future arising.

Freed from the classification of consciousness, Vaccha, the Tathagata is deep, boundless, hard to fathom, like the sea. 'Reappears' doesn't apply. 'Does not reappear' doesn't apply. 'Both does & does not reappear' doesn't apply. 'Neither reappears nor does not reappear' doesn't apply." (Aggi-Vacchagotta Sutta, MN 72)

"If anyone were to say with regard to a monk whose mind is thus released that 'The Tathagata exists after death,' is his view, that would be mistaken; that 'The Tathagata does not exist after death'... that 'The Tathagata both exists and does not exist after death'... that 'The Tathagata neither exists nor does not exist after death' is his view, that would be mistaken.

Why? Having directly known the extent of designation and the extent of the objects of designation, the extent of expression and the extent of the objects of expression, the extent of description and the extent of the objects of description, the extent of discernment and the extent of the objects of discernment, the extent to which the cycle revolves: Having directly known that, the monk is released. [To say that,] 'The monk released, having directly known that, does not see, does not know is his opinion,' that would be mistaken." (Maha-nidana Sutta, DN 15)

So it is unknown, unanswered, or undeclared what happens to the Tathagata after death.