Manually manage music on a PC with Apple Devices app

It was hit and miss, but a year ago I could manually add music to my iphone with the Apple Devices app. But for reasons unknown, it stopped working. I have an Apple Music subscription so I ended up just syncing all my music. However, I did not realize that Apple would replace and delete all my music files. It took way too much time and effort to get everything back to normal, and I don't ever want to go through that atrocity again so I'm trying one more time to figure out how to do what iTunes did with ease but with the Apple Music.

How do I move a song to my iPhone?