Apple Music Files get saved in /Users/your_username/Music/Music/Media.localized/Music/Artist_Name/Album_Name/Song_Name.m4a We can take a back-up from there.

You can export your full library under Apple Music -> File -> Library -> Export Library... as .xml file. Need to disentangle the structure a bit still.

Serato Stems get saved with a 1.2.serato-stems extension and take up about 3times as much space as a .m4a file at 256kbps

Workflow for syncing serato crates is having a handful of playlists in Apple Music that correspond to the same crates in Serato. Then you can Show iTunes Library in Serato drag all files into the crate and untick Show iTunes Library again.

We can keep Cloud and Local Libraries separate but then drop it in the same Serato Crate.

Serato will backup if the last backup is older than a week or if no backups exist. So, if you want to force a fresh backup after making lots of changes, move the current backup into a different location and close Serato.