Keep Music on your Apple Watch
You can store music on your Apple Watch so that you can listen with Bluetooth headphones and leave your iPhone at home for a workout session.
Sync music from your iTunes purchases, Apple Music and iTunes library to your Apple Watch and then choose to listen from your Apple Watch with a long press in the music app.
Choose the playlist to sync to the Apple Watch and how much storage you want the music to take up by using the iPhone Apple Watch app. You can only sync your music when the Apple Watch is on the charger.
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To make this useful, you will need Bluetooth headphones paired to the Apple Watch.