The guy doesn’t want Windows but is ok with Mac. That’s… not how it works. At all.
Mac is based on Linux now
There’s a big difference between UNIX and Linux, and BSD can be very different from even other UNIX distributions. I believe macOS’ userland (definitely not the kernel) is based on FreeBSD 4.2.
The Darwin kernel is based on BSD… sort of. It’s a monstrosity hybridization of an ancient version of BSD and the Mach kernel.
Given the history of A/UX, I’d be surprised if System V didn’t have an oar in the water too.