mjcm_nl wrote:
So I won't be able to go back no more.
If you had music libraries on two computers (a Mac and PC), you'd be able to go back and forth – but each time that you synchronized the iPhone against a different computer, that would wipe all of the music currently on it in favor of the selection from the "new" computer.
So it would be better to pick one computer to be the one that you consistently synchronize the iPhone against.
Is there a version of iTunes still downloadable for MacOS 15.6.1?
No. In Catalna (macOS 10.15), Apple broke up iTunes into a number of applications. These days, you would use the Music application to maintain your library, and the Finder to do manual synchronization with iPhones.
The functionality is very similar to that provided by iTunes – the controls are just shuffled around a bit.