If you chose Automatically, participants will already be assigned to different breakout rooms.
Click the Breakout rooms
Expand the list under Assign participants.
Hover over participant names until checkboxes appear. Choose participants by selecting the checkboxes.
Select More options … and choose a room from the list.
Repeat this process until all participants have been assigned to rooms.
To view the participants assigned to a Breakout Room, expand the arrow next to the room's name.