I work on an open source project in my free time. Officially we support Linux, Windows, and macOS.
I had to change ~2 lines of code to port the Linux/Mac code path to FreeBSD. Windows has a completely different code path for that critical segment because it’s so different compared to the three Unix/Unix-like.
This is a very specific example from a server side code that leaves out a lot of details. One being that we wrote our project with the intent that it would be multi platform by design. Game software is wildly complicated compared to what we do. The point here is that it should be easier to port Unix to Unix-like compared to Unix to Windows.
I’ve heard that PS3 games, while the OS was too, I think, based on FreeBSD, ran in a sort of a hypervisor and used some features of the Cell architecture.
I’ve never read about PS4 and PS5 OS’s, and them being Intel-based should mean that it’s possibly less exotic.
Of course various unices are almost source-compatible.