As far as I know, to a large extent the soul stuff is canonically uncertain.
Some people have had visions of their past lives. This does not mean ALL souls reincarnate, or that new souls are generated, or whatever.
The Brass Coast think some end up as dolphins. The Marches think some end up as apple trees. The heresy of the Land Without Tears (entirely player made, driven, combatted and condemned) suggested a way out of the reincarnation cycle altogether. And it’s suspected that dead claimed by the Vallorn have their souls trapped within them.
As for what an Eternal does with a human soul… ah, you’ll have to ask around
And then there’s the Orcs, who I still don’t understand…