When the spell Banishment is cast:
If the target is native to a different plane of existence that the one you're on, the target is banished with a faint popping noise, returning to its home plane. If the spell ends before 1 minute has passed, the target reappears in the space it left or in the nearest unoccupied space if that space is occupied. Otherwise, the target doesn't return.
Unlike the case where the target is native to the plane, they are not incapacitated for the duration of the Banishment. That means the target could cast a spell, like Plane Shift, to return to the plane they were banished from.
If they do this, and return to the plane they were banished from before the end of the Banishment's duration, do either of the effects at the end of Banishment ("reappears" if the spell ends early, or "doesn't return" if it doesn't) have any actual effect, given that the target has already extracted themself from the plane they were banished to?
Best Answer
If the spell ends, the effects still occur.
There is no evidence that the spell operates any differently if the creature affected moves to a new plane.
If the spell ends before one minute is over, the creature will return to the space it left (or the nearest unoccupied space). It would essentially teleport to where it started on the caster's plane.
If the spell goes the whole minute, the creature doesn't return to the space it left. I.e. it stays wherever it is on the plane it plane shifted to.