Transfer music from iPad to iPhone

The master copy of all music is iTunes on your computer. Music purchased from iTunes can be downloaded again from your phone, and actually is automatically when you transfer content from one device to another. Music from other sources (MP3’s and ripped CDs) must be first added to iTunes on your computer, then synced to your phone via USB cable.

There are two other ways for non-iTunes music: Apple Music and iTunes Match, a paid service that uploads your music to iCloud, where it can be synced to your phone from iCloud. Here’s additional information→If songs are missing from your music library after you turn on Sync Library - Apple Support

Can only be done with third party apps and even then only if you own the music.

Apple music sync is always computer to iDevice