Question On Lore

What happens to Demons after they are killed?

Angels are simply reborn through the crystal arch. That was pretty easy to find.

But what about demons? Is it different for regular demons, great evils, and prime evils?
Is it a phylactery type thing where they need to store part of their soul?
Or is their ‘essence’ immortal and they just eventually return back into existence?

I couldn’t find much info on it.

How do these differ from Nephalem, since they are both Demon and Angel?

I thought this sounded vaguely familiar…