How can I transfer music from my iPhone to my iPad?

Ideally all of your music would be in an iTunes or Music library on your computer, running a compatible version of macOS so that it could talk to each of your devices. See Recover your iTunes/Music library from your iPod or iOS device - Apple Community should it be necessary, however the easiest workaround may be a subscription to the Apple Music service, merging the music on your iPhone into an iCloud Music Library which you can then access and update from your other device and computer. If an ongoing subscription doesn't appeal you should be able to download your music to the iPad and turn off the Sync Library option before cancelling the automatic subscription renewal.

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See Subscribe to Apple Music on iPhone - Apple Support. There should be a prompt when you join asking if you want to keep your music or replace the content on your device with your iCloud Music Library. You must select Keep Music at this point.

Music on the iPhone should then attempt to match or upload each of the tracks on the device, and add all of your playlists to your iCloud Music Library. These then become available on your computer and your iPad once you enable Sync Library AKA iCloud Music Library there. You can stream content on demand, or download it to the current device so it can be used offline. Your own purchased, matched or uploaded content that has been downloaded to the device will be retained if you decide to cancel the Apple Music subscription, anything else that you have added via the Apple Music service will be deleted.

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