When you add music to your Apple Watch, you can listen to it wherever you go, even when you don’t have your iPhone with you.
You can add specific playlists and albums to your Apple Watch using the Apple Watch app on your iPhone. If you’re an Apple Music subscriber, you can also add music with the Music app on your Apple Watch.
On the Home screen, navigate to the music you want to add.
Or, from the Home screen, tap Adjust Apple Watch during a workout