syncing ny music to an iPhone 12

The iTunes version on previous macOs versions should still support the iPhone 12, but may need a software update which should be offered when you connect the device to the Mac.

If you are using a later version of macOS, then you can use the Finder as explained here: link-> Use the Finder to sync your iPhone, iPad, or iPod with your computer - Apple Support

Follow the steps here: link-> Use iTunes to sync your iPhone, iPad, or iPod with your computer - Apple Support

This support doc is for iTunes; the indication of an iPhone 12 suggests a later Mac OS than Mojave as well, which puts iTunes out of the picture... rendering this "help" doc essentially useless for this topic (it was for me).