[musicale]

Apple seems to have a long history of integrations of Mac OS (and the Mac user interface) and UNIX, starting in the 1980s and continuing through today: A/UX, AWS (Apple Workgroup Server), MAE, Rhapsody, OS X, modern macOS...

Not to mention the OS X-like variants for their other platforms: iOS, tvOS, iPadOS, watchOS...

Also consider the various Mach implementations on Apple hardware: MacMach, MachTen, MkLinux, OS X/Darwin...

Sadly it seems that the classic Mac API died when Carbon and 32-bit apps died, so you can't easily port old Mac apps to modern macOS. macOS (and iOS) seem to have a rather poor record for backward compatibility, and it's only going to get worse with the current architectural switch to Apple Silicon...