How do I transfer my music library from my iPhone to my iPad.

Put the music on your iPad the same way you put it on your iPhone. I assume you are using the same Apple ID on both devices.

If so, and you have an Apple Music subscription:

Turn on Sync Library with Apple Music - Apple Support

If you purchased the music from the iTunes Store:

Redownload apps, music, movies, TV shows, and books - Apple Support

Or, if you synced your iPhone from your computer:

Sync iPad with your computer - Apple Support