Unity uses the Umbra library to perform occlusion cullingA Unity feature that disables rendering of objects when they are not currently seen by the camera because they are obscured (occluded) by other objects. More info
See in Glossary. Further information on Umbra, including information about the bake process, the data structures inside the occlusion culling data, and the runtime operations that Umbra performs, can be found in the following articles:
This Gamasutra article by Umbra goes into detail about how Umbra works: Next Generation Occlusion Culling by Umbra