As this Answer states from the question Can summoned creatures take items back with them?
The FAQ answers it (p.91)
What happens to a summoned monster’s equipment when it dies or the
spell ends? What if it’s not carrying the equipment any more? What if
it’s carrying something new?
When a summoned creature goes away, so
does everything it came with. If it’s holding or wearing something it
didn’t appear with, that item drops to the ground in the creature’s
space after it disappears.
If we only consider summoned monsters with a temporary duration spell, I believe the same thing would happen if the creature dies, the spell ends,if you dismiss the spell,use dismissal,banishment or even dispel magic on them.
However for a creature that is on the material plane (that wasn't summoned with a temporary spell) targeted with the spells Banishment or Dismissal it seems like there is no official ruling about it, I guess all their equipments goes with them (I don't know about newly acquired equipment that could be another question on its own) but since only extraplanar creatures can be targeted they would not bring the other creature with them, unless it is also another extraplanar creature (and that you also would try to target with Banishment).
But I think you can't target the swallowed creature though because it would benefit from total cover and total concealment as well.
Magic works in mysterious ways... it does not always make sense...