Putting music from MacBook to XR

To sync music from a computer to an iPhone, you have to add the music you want to iTunes first and then sync.

Sync your iPhone, iPad, or iPod using iTunes on your computer - Apple Support

Have a read through this (old) article, I believe the process still works as long as you have “Manually manage Music and Videos” ticked under the “Summary” tab for the device and the file is iPhone compatible. I can’t say whether having an Apple Music subscription causes any conflict but it’s easy enough to try out and see what happens.

http://osxdaily.com/2013/08/26/copy-music-directly-to-iphone-ipod-without-adding-to-the-computer-itunes-library/

Although you still employ iTunes for the process, it should allow music files to be dragged from Finder to the iDevice under the iTunes Sidebar without the necessity to import (it did when I last used it but that was quite some time ago).

Hope this helps.