How do I get my music back from iTunes which I don't have on my MacOS?

Much we need to know, you haven't told us. The most important fact is: your iTunes stuff is/was on the same computer you're using now? If so...

• open the Music folder in your user Home folder; in there you should see iTunes. *IF* Apple Music will convert old iTunes libraries, then launch Music and in its preferences point to your old iTunes library (or try holding down the option key when launching Music which will ask you for a library to open).

If not, and assuming you have a backup of your old computer...

• follow the advice given to you by Limnos by copying the ~/user/Music folder from your backup to the same location on your new machine.

iTunes was rebranded the Music application. Open your Music application and your library should be there.

restore your Music folder from your backup.

Always back up your media files. Do not rely upon Apple keeping a copy for you. Redownloading is a convenience but not guaranteed. If a distributor decides to pull an item from the store it will no longer be available. One on a backup will be.