Synching with iTunes

If you want to sync over USB then Sync Library (AKA iCloud Music Library) should be switched off. If you want to use the iCloud Music Library features of Apple Music that allow music and playlists you add to your library or device to automatically sync over the air then you want Sync Library enabled in each of your libraries and devices.

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When you enable Sync Library on a device so you can use Apple Music you are no longer able to sync over USB. See Use Sync Library to access your music library across your devices - Apple Support for details of how to turn this option on or off.

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When you enable Sync Library on a device so you can use Apple Music you're supposed to enable Sync Library on a computer too so that the music and playlists can sync in either direction between library and devices via the cloud.

Have you signed in with your friend's Apple ID, or have they added you to an Apple Family Sharing group with a family Apple Music subscription? The first is not a good idea, and the second has possible negative implications for whoever is the family organizer in terms of paying for other people's purchases.

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