Unity uses the Umbra library to perform occlusion cullingA process that disables rendering GameObjects that are hidden (occluded) from the view of the camera. 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