Here is a list of rooms on the right. Each room is listed on a separate line. If you click on the name of a room and there are fewer than 50 people in it, you will be able to join that room.
On mobile devices, accessing the list of rooms is a bit more complicated. The small screen size does not allow for the entire list of rooms to be displayed at once. Therefore, there is an additional button that you can click on to open the list of rooms.