I do not think so there will be any satisfactory answer to it until one can start making any effort in 8-fold Path. What happens after Nibbana? Nibbana of course.
The closest explanation I can give is that Nibbana becomes the dominant “layer” before anything at the stage Arahanthood but before that other aggregates can be still dominant like in Sotapanna stage because of yet to be resolved avijja.
As Lal said: “Nibbana is the “state” that is reached when all those are stopped from arising. There are no citta, cetasika, rupa “in Nibbana””