Comment by herpdyderp

There's this really old product that Apple use to sell for all your music. I think it was the youPod or something...

You can sync your own MP3s using iTunes Match -- it's about $25/year. https://support.apple.com/en-us/108935

I use streaming now but historically I had no issues syncing music and playlists to my watch.

I wish I could do this but I have yet to find a good Apple Watch replacement for owning and syncing music (rather than streaming it)

Is it not possible to sync MP3s to Apple Watches anymore? I have a really really old model, and I selected a few playlists on my iPhone, and when they change, the songs automatically sync to my Apple Watch.