Your iPhone and Watch need to be in close proximity for music to sync, so you can't leave your phone behind until the sync is complete. Watch versions earlier than watchOS 9 will need to connect the Watch to power.
How to add music using your Apple Watch
Apple Musicsubscribers can also add music directly from their Watch. While you can't use this method for Apple Music Voice, this method allows you to download music from Apple Music to play on your Watch without the need for Wi-Fi or cellular data.
Apple Music subscribers can add music from their Apple Watch.
- Open the Music app on your Apple Watch.
- Tap Listen Now or browse your music to add to your library.
- Select the playlist or album you want to add to your Watch and tap the three dot button.
- Tap Add to Library to add the music to your Watch.
- Tap the three dot button again and select Download to play the song on your Watch without an internet connection.
Apple limits the number of songs you can sync with your Apple Watch to 2GB of about 250 songs.