It depends on the definition of UNIX, but typically Linux doesn’t count as UNIX.

You can draw a continuous—if very torturous—line between Bell Labs’ UNIX original codebase and modern macOS/iOS.

Strictly, no - the only vendor whose systems are currently certified to meet the UNIX trademark and Apple (MacOS), IBM, HP, and SCO. And on practice the user interface and APIs are so different it’s much more accurate to say if has some UNIX-like parts, but isn’t UNIX. - ditto the iOS family.

No, because the userspace is based on Java and the NDK uses Android specific APIs, only ISO C and ISO C++ standard libraries are officially supported, anything POSIX related is not part of NDK official APIs.

In rooted devices everything goes, on random Android device you application might get killed for trying to access private APIs, meaning anything that isn't documented as official NDK API.