There are two ways you can control and listen to music using your Apple Watch: Either with a synced playlist on the Watch itself (using a pair of, say, Bluetooth headphones) or through the songs stored on your connected iPhone. Choosing between those is simple; just press firmly on your Watch face when you’re within the Music app, and you’ll see a “Source” option.
Once you’ve chosen where you want to play music from, then you can pick where to send the music to. If you selected the Watch as your source, it’ll automatically attempt to play your tunes through any Bluetooth speakers or headphones you’ve paired.
If your iPhone is the source, though, you can press firmly on the “Now Playing” screen and tap “AirPlay” to see your output options, which’ll include any Apple TVs or AirPlay speakers you’ve configured.
That’s nifty! Isn’t it kinda weird, though, to use a watch to control an iPhone’s music that’s then playing on an Apple TV? That makes me feel both futuristic and somehow very, very lazy. I mean, I am lazy, but I don’t like to be reminded of it.