LIVE classes are a one-way camera meaning only the viewers can see the instructor and the instructor only.
LIVE Open-door classes are a two-way camera meaning, the instructor can see the participants, the participants can see others in the class, and see the instructor, (with the ability to turn off their camera.)
You can find a list of available both LIVE options on the home page about a fourth of the way down.
You can join classes 5 minutes before they start or anytime throughout the duration. For a class that hasn't yet begun, instead place of the "join" button, you will see the option to add to your schedule.