Nibbana has nothing of this world.
Existence in assana realm is exhaustible with end of kammic energy sustaining it.
One dying there, one may reborn anywhere unless any of eight ariya worthy achievement was had by that lifestream.
Nibbanā ≠ Samsarā
as claimed by some teachers.
Four fundamental realities are cittā-cetasika-Rūpa-nibbanā.
Former three are called saṅkhata. They are dangerous.
Nibbanā is only asaṅkhata and true refuge.
Former three (cittā, cetasika and Rūpa)
And fourth one (nibbanā) are mutually exclusive.
Nibbanā is complete absence of suffering after parinibbana. Eradication of suffering caused by samphassa-jā-vēdanā during last jati in case of Arhant.