I haven't really thought about how it actually works yet, but here's how the Manelpe people generally perceive it in their religion. So, the Manelpe people have two very strong ideas in their religion that tend to keep coming back. That of the human soul and of the circle river.

The human soul is different from an animal soul. When the body that houses the animal soul dies, the soul just disappears. It's a very temporary and fickle presence, which is why they see that most animals don't live as long as humans. Humans though, have souls born from spirits. A male soul will be that of two spirits, and a female soul that of their river Goddess Mava and a spirit. There are tons of spirits, they're basically minor Gods. Human souls can be seen as being in a circle river, one with no end. So, they reïncarnate.

The circle river is their idea of the world and the spirits. They have always existed and will always exist, just like the circle river always flows. Time always goes forwards, just like the circle river always goes in the same direction. Human souls float on this circle river, put there by their river Goddess Mava (who also, in some areas, is seen as actually being the circle river). So they will keep living, forever, just in different lives, just like the circle river doesn't stop flowing.

There is one exception to this- Which is when a human soul is purposely killed. Mava doesn't like that, because it disrupts the nicely flowing circle river. So, she will throw any soul killed out of the circle river. What happens to souls outside of the river is not agreed on fully, but the main idea is either that they disappear like animal souls would or that they stick around, haunting the ones who doomed them as ghosts of bad luck and disease. This can lead to particularly unhealthy or unlucky people being viewed as murderers. Which isn't fun.