Background: in one of our campaigns we have enough Elementals that are being formed in the material plane, instead of being formed in an elementary plane. The elementals are identical to those that appear in the Monster Manual.
We are reaching the levels where some of us can choose the 4th level spell Exile like a spell I was wondering if to cast Exile in an elemental formed in the material plane a) it would banish it to an elementary plane and it would not return b) it would only be banished temporarily to a demiplane and would reappear in the material plane when the spell ends.
The way I see it is that it would have to do with the interpretation of what is "airplane native" and "airplane of origin" in this case.
The description of the Exile the spell (PHB, p. 217) says:
You try to send a creature that you can see within reach to another plane of existence. The target must succeed in a Charisma saving throw or be banished.
If the goal is native of the plane of existence in which you are, banish the target to a harmless demiplane. While there, the target is disabled. The target remains there until the spell ends, at which point the target reappears in the space it left or in the nearest unoccupied space if that space is occupied.
If the goal is native from a different plane of existence to the one you are, the target is banished with a slight burst, returning to its plane of origin. (…)
I suspect that he would banish these elemental creatures from homegrown to a demiplane in the case of our campaign; unless we were in another plane, in which case it would be banished to the material plane since this is its "plane of origin".
There is this question that gives a kind of answer:
What determines the native plane of a creature for the Exile spell?, But I would like a more concrete answer if possible to the scenario presented above.