How does iCloud music determine which songs to sync to iPhone?
What Apple Music iCloud Music Library does is the following:


It will check all the tracks you have stored in iTunes and
See if they are available in the Apple Music catalog. If not, Apple Music will
Upload them to the cloud. 
All these songs will be available from the Music app on iOS. If you play them, you will stream them from the cloud. If you would like to save songs for offline listening, tap the triple dots next to a song, and tap: Make available offline. 


Because it works this way now, it is no longer necessary to sync via iTunes