By default, participants are automatically moved into breakout rooms when they're opened. When you turn off this option, participants receive a message asking them to join a breakout room. They select Join room (Join on a mobile device) before being moved.
In the meeting controls, select Breakout rooms
.Select Room settings
.Switch the Automatically move people to rooms toggle off.
Select the back button < to save your changes.
Meeting attendees can learn more about joining and participating in breakout rooms here: Join a breakout room in a Teams meeting.