Basically, the current state of affairs with souls and the afterlife multiverse wide is that it's literally a game of bobbing apples, but with souls, with various people trading with them dependant on the forms they take.
The abyss has Larvae, which they peddle and sell on a particular layer, that make up part of the Demon Diet and ecology, Demogorgon himself knows a way to turn any aligned soul into one of these, thus, make them a demon.
the 9 hells have their own version of this which is this fucked up merger of pre-gestated wombs stuffed in their devils called Ovatorium/it being halfway mixed with subverted Baatorian reproduction, which get's even more confusing because one is effectively a devil if "Considered to be one in alignment with Asmodeus." Hence Erinyes, and various fallen Celestials present which weren't apart of the promotion/Demotion system they have there.
Limbo and the like a of the NE planes lso have larvae, though I think the difference is that the larvae there have arms, opposed to heads on maggots, and Night Hags, and various other hags trade them.
I should note that Limbo has Entropic Reapers (Libris mortis) to also collect souls
Vecna is also in the soul-business, having his own undead repears that capture stragglers for whatever, I'd have to talk about soul-fuckery involving certain classes, spells, and magical items to further this, so I'll leave it here.
The bulk of souls take on whatever forms they do as petitioners in the event they followed a deity, and the default ones go to Kelevemvor, who hates his job because he can't do dick with it, and his predecessors fucked him over with the dullests most depressing afterlives after,
Basically, if you didn't opt in to D&D esque spirituality the following happens-