how to transfer music from iphone 7 back to Mac

If I'm understanding the question correctly, it seems that you have music on the old phone that (possibly) did not come from the iTunes Store, but was synced to it from a computer, but you no longer has access to those files, and you wish to transfer them back from the old phone to your computer... If that is the case DO NOT sync your phone with your computer, because your phone will only have music on it that is currently on your computer, and you will completely lose the songs on the phone that are not already on your computer.

Community user Turingtest2 has gone to the trouble of describing how to go about this in detail.

Recover your iTunes library from your iPo… - Apple Community

Hello Johnb-one,

Thank you for reaching out to Apple Support Communities.

You would need to sync your new iPhone with the same library you synced your iPhone 7+.

Another option would be to enable Apple Music on your iPhone 7+ and enable Sync library:

"Turn on Sync Library

With the latest version of iOS, iPadOS, and macOS, Sync Library is turned on by default. If you turned off Sync Library, you can turn it back on. Just follow the steps below for your device.

On your iPhone, iPad, or iPod touch

1. Go to Settings > Music.
2. Turn on Sync Library. If you don't subscribe to Apple Music or iTunes Match, you won’t see an option to turn on Sync Library."

More details can be found here: Turn on Sync Library with Apple Music

If you are not able to set up Apple Music on your iPhone 7+ can yo provide a few more details? What happens when you attempt to sign in and set up Apple Music?

Cheers!

My pleasure, but the real credit goes to Turingtest2.