Hello Michèle
Yes, you can load your music on it to play music directly from the watch and leave your phone at home. You just need Bluetooth headphones that you connect to the watch. There's a Spotify app for the watch that's supposed to make transferring music easy, but you seem to need a Spotify Premium account (because of the offline music). I haven't tried it out yet though. When the watch is connected to the mobile phone, you can control Spotify at any time, regardless of whether you have a premium account or not.

Hello Michèle,
Besides Spotify, Amazon Prime Music also works and as far as I know there are other apps. You can install them all via the Connect IQ Store (app store for Garmin wearables).