The _UID tag in GEDCOM is a user-defined tag because it starts with an underscore. What that means is that it is not a tag defined in the GEDCOM standard, but is one that some vendors added for their program.
The purpose of the UUID is to identify a record uniquely across any system. Some genealogical vendors may have wanted to add a unique identifier for their own purposes. The first one to do so (I'm not sure who that was) may have named the custom tag _UID and other vendors may have followed that lead.
Each vendor (as named in your quote) defined the tag in the way that they thought was right and/or the way they wanted to implement it. There was no standard to follow, and they were free to make up their own structure, possibly reading up about UUIDs and trying to follow how they have seen other vendors already implement it.
If you are lucky, your program's vendor may have documented their implementation of the _UID tag, and that may give you something.
The INDI's xref_ID uniquely describes an individual within a single GEDCOM, but they don't have an official mechanism to connect them between GEDCOMs.
They did include the "Permanent_Record_File_Number" defined as:
which they said "will be used in the future as a network reference to a permanent file record". But as far as I know, no developer ever included that functionality in their programs, and chose to use the UUID in the form of a _UID tag instead.
So you could then for your needs, reference GEDCOM records using that structure, with the GEDCOM file name as the "network reference".