The top header in your online classroom shows the organisation icon, room name, followed by the space you are in currently (i.e. either Main Space or the name of the breakout team you are in). You can also see how many people have joined the Main Space or your breakout team.
If you have permission to join other breakout teams on your own, click the small blue dropdown arrow next to Main Space to view all teams and the number of members in each team. Then click the name of the team you wish to join. You’ll be moved to the selected team instantly.