Hi ​@Mofofunk, welcome to the Sonos Community!

Google Assistant will use the default music service that is set in the Google Assistant app, unless you specify otherwise (“Play ‘Song Name’ on Spotify” for example). 

The list of supported music services depends on your region, but you can change your default service by following the steps on Google’s guide for linking and setting your default music service. Our support article for Controlling Sonos with Google Assistant has additional information, but the Google article has the steps you need.

I hope this information helps!