Apple rebranded "Mac OS X" to "OS X" and later rebranded that to "macOS". It's not like they're different lines of operating systems; it was a rename of the whole line, so my impression is it's fine to use the term "macOS" to refer to any of the versions since 2001, or it's also fine to use the name that was given at release when referring to a specific version. In other words, probably best to not worry about any particular phrasing, and not try to put exact technical meaning on any of these terms. :-)

For example, Wikipedia[1] has a page called "OS X Yosemite" which describes it as "A version of the macOS operating system", and the Wikipedia article on macOS[2] says it was first released in 2001.

[1]: https://en.wikipedia.org/wiki/OS_X_Yosemite

[2]: https://en.wikipedia.org/wiki/MacOS

>I don't understand why people

Because it's a bother to keep in mind when they introduced the new name? Or release names? It's hard to imagine that someone would actually be talking about certain number of releases this way.

PS: Apple's stupid marketing bs strikes again.