I experience the same problem on a new iMac with MacOS 11.1 (Big Sur) and an iPhone XS with iOS 14.3. In contrast to BGillings, I do see the iPhone in the System Report and Dropbox can sync images (i.e. it obviously is recognised as USB device). But unfortunately the Music App does not recognize the iPhone.
The iPhone is still properly recognized and can be backed up with my old MacBook Pro, which still has the iTunes App. From my point of view the problem seems to be either Big Sur or the Music App.
Has this problem already reported by more users? Is there already a solution? I appreciate your help!