Worse yet it's the only effective way to sync music to the iphone over a wired cable. I can tell they've abandoned their USB sync software because of how annoying the dialogs are when you plug your phone in and don't authenticate to start it, additionally that thay've (likely intentionally to push I-cloud) never added automatic usb photo sync from phone to computer. This probably all originated from when Apple shifted their convergence model from all around the desktop to all around the phone¹
The iPhone (and previously iPod), from day one, could never simply be mounted as an external USB (or networked) file system. You always had to do the ridiculous "sync" song and dance and use their proprietary software, instead of just copying files using the OS like every other external storage device. Maddening.
more maddening yet is you can actually mount it as a limited storage device using https://libimobiledevice.org/ under linux, but not MacOs(to my knowledge)
How do I copy music to my device?
Sorry, music synchronization with newer devices is currently not supported but if you are a keen developer why not contribute a new service implementation for the ATC Service?
[1]https://www.youtube.com/watch?v=b9_Vh9h3Ohw