New Phone asks for Other Apple ID login for iTunes
How do I either transfer old songs that I got third hand, or leave those songs behind, but stop receiving messages asking for other people's Apple IDs and passwords?
iPhone 6, iOS 12
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Music purchased under someone else's ID means they're not yours. To avoid this prompt, you would need to eliminate those items from your iTunes library, or at least from your device synchronization to your library.
In iTunes, you can see the ID used for the purchase if you select "Song Info" and go to the File tab. This can only be done one song at a time, and there is no way to bring the ID into view any other way (not in the library view, nor smart playlist). However, you can create a smart playlist of Purchased songs, then open Song Info on the first one and scroll through each song while viewing the File tab looking for songs that aren't yours (delete them as you go).
If you have unauthorized songs, you may also have unauthorized apps too. Those will create the same prompt for passwords of the other ID. Since apps are no longer in iTunes, you can't audit this in any way to identify them. You may have to restore (erase) your iPhone to start over with a "clean" set of apps that are yours.