How do I change the groups I'm playing within the Sonos App?

I have several different groups set up on my Sonos system (multiple Sonos Ones).   In the Sonos App, I would like to be able to select which group to play and/or to switch between groups (e.g. sometimes I want to play music in the living room, but nowhere else.   In short, I’m simply trying to figure out how to play music in different rooms at different times thru the Sonos App.  I assumed I could just switch the group that was playing, but I cannot make that work.)   Can someone tell me how?

Thanks