Transfer music from iphone5 to iphone12

For music purchased directly from the iTunes Store, see here: link-> Redownload apps, music, movies, TV shows, and books - Apple Support

For Music synced from iTunes on a computer to the iPhone 5, then you will need to sync them to the iPhone 12 from the computer. If the music is no longer available on a computer to sync from, you will need to extract it from the iPhone using a 3rd party app, add it to an iTunes library on a computer, and then sync it to the iPhone 12.

See here: link-> Recover your iTunes library from your iPod or iOS device - Apple Community

Your music that is on the phone should be in your iTunes library. You transfer music like that. There is no way to transfer music directly from one phone to another.