Umbra is a pro feature, ok. So, what kind of occlusion culling is on in the free version? I suppose, there simply must be back-face culling on by default, right? Is there Z-buffer overdraw culling for curbing fill rate?
Yep.
Well it’s more for providing scene correctness than about saving fillrate because you’re still running almost all of the graphics pipeline even for skipped fragments, but yeah, Unity uses Z-buffering.
The other main technique it uses is frustum culling - objects that are behind or to the side of the camera’s viewable area are skipped by the renderer.
(None of these are ‘occlusion’ culling, which is specifically about skipping objects that are hidden by other objects).