UI is generally rendered in screen space and is not affected by PP at all. You can do a world space UI (used in lots of VR games and occasionally in some games eg:
If you do a world space UI, then it’s rendered to a texture and it will be affected by PP just like everything else.
There’s also the case where you want to overlay something 3D onto the UI (simple example is an RPG inventory system):
If you want PP on that, it’s not well-supported in URP, and there’s a whole thread about it here: Post Processing with multiple cameras is currently very problematic .