I got my 645M a few days before the Spotify app was released, but didn't have any BT headphones yet. I went ahead and followed directions I found online to load some of my iTunes playlists from my computer onto the watch, I could find them easily on the watch, even if I couldn't yet play them. Then the Spotify app was released, so I loaded it and copied over some playlists, and on the same day, finally bought some Airpods (works 99.8%, posted my experience on the headphone sticky thread) and was enjoying Spotify from my 645 ...but couldn't find where on the watch my iTunes songs where located for the life of me, even though I could see them in Garmin Express. Searched this board and other online sites and still couldn't figure it out.
I just happened across another post on this board, several months old, which mentioned changing "libraries" in the general settings > music rather than in the music app itself under what it calls "Libraries." Eureka! But also, now, very confused on why it's set up this way. Makes it difficult if not impossible to switch between Spotify and my iTunes songs on the fly. Seems like a miss on the development side.
Just posting this in case anyone else is in the same boat as me and having trouble finding/accessing songs they've loaded from the computer (not Spotify) to the 645. It's not intuitive. Also, add me to the list that would love to see this changed in a future update.