Got a question about the end...*spoilers ahead*

How could anyone understand it when it's not a real thing? There is nothing to understand, it is basically magic. It doesn't matter what happens to the soul because it doesn't matter from the perspective of the movie or the story they are telling.

I think it's pretty clear that when our characters die they become ghosts, likely like the ghosts that died during the original ceremony. It's also clear that the ghosts can somehow inhabit bodies. It was clearly stated and shown that the souls were leaving the bodies and that our physical forms were nothing but shells. So one person dies and becomes a ghost and their physical body becomes a shell that another ghost can inhabit. Maybe I'm not understanding your question but I thought this was explained perfectly well in the movie.

What is "real"? To an atheist no such thing as mentioned above could exist but there's some crazy ESS in the occult world..

At the end Jim talks about needing new vessels from time to time and plenty of souls to eat. That is all i know about it.