Well, ‘nix-dirived, actually, but yes you are essentially correct. Except that I do not believe that Linux will run any OS-X programs.
I could be wrong. Not really familiar with macmachines, I must admit.
No, OS X absolutely is UNIX. It’s been certified to high (highest?) levels as such, a very expensive & demanding process - which Linux _hasn’t_ achieved.
You’re probably confusing the OS with the window manager, bundled as part of OS X distribution and (exact details escape me) not particularly conducive to running on a non-Apple configuration. The core problem is there’s a bunch of non-OS stuff required for most OS X programs, and if you don’t have the libraries/source/etc making it run will be a major pain. [does quick search] Ah, there is a project to run unmodified OS X binaries on Linux: https://www.darlinghq.org but it’s going slow.