This is not an exchange of information. Your smile is more important. The verbal is less important than the non-verbal. Also, "thank you" or "thanks" helps: "I'm fine thanks". Also, you don't have to answer the question. You can ask your own question or say something else: "Hey, what's up?"

In America, we tend to answer over-positively, so "great" might equal "good," good = ok, and ok/fine = not that good or neutral. "Not bad" is very common for a neutral expression, especially if said in an upbeat sort of way. It may depend on intonation and body language-- sometimes people can say 'fine' or 'ok,' and it actually implies they aren't doing great. If a person answers "fine" and they seem irritated or sad, sometimes they might expect you to ask "is everything fine." Sometimes we call that "fishing;" when a person answers "fine" or "ok" and they're trying to get you to ask them if everything is fine. Some of what you saw in movies is probably related to the above. Of course, it also varies by personality, energy level, gender, etc.

Here is an example of "Good". https://youtu.be/UZCPeQhpcTs?t=584

It is a typical response. You are correct. Usually I say just 1 word. "Fine". Or I say "Good". Only 1 word.

I think it’s a typical answer, especially when someone asks how are you as a formality, just to start a conversation. I would say in movies they don’t include much small talk because that would be boring, that’s why they don’t just say I’m fine or I’m doing good.