"UNIX®, an open standard owned and managed by The Open Group, is an enabler of key technologies and delivers reduced total cost of ownership, increased IT agility, stability, and interoperability in hetero¬geneous environments enabling business and market innovation across the globe." Details of those specs can be found here.
Curiously enough the latest standard listed on their website is UNIX 03, and to quote another source, "UNIX® 03 - the mark for systems conforming to version 3 of the Single UNIX Specification".
Based on the above sources, it seems UNIX nowadays is not that specific OS, but rather a standard derived out of the best possible generalization for how operating systems in Unix family behave.
The Forum is also the specification authority for The Open Group UNIX certification program. The Open Group works closely with the community to further the development of standards conformant systems by evolving and maintaining the value of the UNIX standard.
This includes making the standard freely available on the web, permitting reuse of the standard documentation in open source projects, providing test tools, and developing the POSIX and UNIX certification programs.
In a very specific definition, mac OS version 10.13 High Sierra on Intel-based hardware is compliant to the UNIX 03 standard and to quote the pdf certificate, "Apple Inc.
has entered into a Trademark License Agreement with X/Open Company Limited." Side note: I hesitate to question what it would means for mac OS 10.13 on non-Intel hardware to be treated as, but considering that hardware is mentioned for other OS, the hardware is significant.
Although patches can vary in size from several kilobytes to hundreds of megabytes, patches are usually perceived as being rather small.