What do you call the main area in an auditorium below the balcony? I am thinking "main floor" or "first floor", but it sounds not specific enough and I think there might be a better word for it. What would you call it?
In a theatre it's called the "stalls". Ref: https://theatre.london/theatre-guide/beginners-guide-sit-theatre/
In theater's that put on musicals or plays it would be called the "Orchestra" Section
2b: the forward section of seats on the main floor of a theater
2c : the main floor of a theater
But, in auditoriums like this one that wouldn't apply since there is no orchestra pit. I am not sure there is a generally accepted word, but there are common terms (including the ones you suggested) such as:
Ground Floor (typically only if the main floor is on the same level as the ground)