[tgv]

One thing that makes macOS stand out, is the frameworks. Every version of the OS has come with improvements in the UI libraries. I remember being amazed at Cocoa's bindings, which developed into a way to bind arbitrary data to UI controls. It was reactive before React (although not taken that far, because the UI 'language' was limited). That's just one small part, and it's why replicating macOS is so hard.

[pjmlp]

Just like any other desktop OS since 16 bit home micros, with exception of MS-DOS.

BSDs and Linux distributions are the exception with their fragmentation, even other commercial UNIXes had a proper set of desktop frameworks.