Darwin OS (the unix os that Macs run on) is a weird hybrid of various code from multiple older BSD flavors, though I hear it’s perhaps most similar to FreeBSD, with a unique kernel developed at Carnegie Mellon. An old friend from high school went to CMU and got to work on it. He works at Apple now, and still does kernel and driver development, among other things. Prior to working at Apple, he was doing Linux kernel development for years. Given his proprietary knowledge, he probably is not allowed to contribute anything to a project like that anymore.