Ok, so the magic word "это" is used in Russian to denote two different things: "it" and "this/that" and I think that is a primary source of confusion.
The word "это" implies the neuter gender. In your example above, "Что [это] такое стадион?" the girl is literary asking: "What [kind of thing] that [is that you call a] stadium?" So, one of the possible answers to such question would be: "Это стадион." ("This [is the kind of thing that I call a] stadium.")
Hope it helps.![]()