As the title says...

How do you do this?

I have some music in my iphone that I downloaded from computer files of disks I copied years ago. How can I share these with my girlfriend ?

They are in my Apple Music app and I can play them of course but I can’t share them to her device.

Create a playlist in iTunes and put as much music as you'd like on it.

Connect the other iPhone to your computer, use iTunes to copy the playlist to this phone.

I recently did this with an old iPhone 5S that I basically want to use as an iPod. The playlist has nearly 700 songs on it.