What does “you got it” really mean here?

Why is join in this sentence in the past tense?

There are two possibilities here! Either the clip was filmed earlier in the day, so he uses "joined" in the past tense. Or, they're talking about his actual physical movement from London to their current location. That movement, or "joining," would be a past action. Ex: Here's a clip from earlier in the day, where Barbara joined us to talk about the ocean. Ex: I joined my family here in London, after staying in Wales for a couple of weeks. Ex: Now, joining us live from London, is Sarah.

What does the word "pit" mean in this context?

A pit is an indentation or hole in the ground. Thus, an orchestra pit is similar! In the theatre, it is the area where the orchestra plays, in front of the stage. It is usually on a lower level than the audience, thus it is called a "pit." Ex: A musician fell over in the orchestra pit. Ex: The music coming from the orchestra pit floated up to the upper levels of the theatre.

What is the meaning of "being someone's right hand"?

"Being someone's right hand" refers to being someone's reliable helper. This idiom comes from the idea that most people are right-handed, and therefore their right hand is "stronger". Ex: My mom is my right hand, she helps me with everything. Ex: I'm my manager's right hand, I help him with many tasks.

What is the difference between "sustain" and "maintain"?

Great question. These verbs have similar meanings. To "maintain" something means to "upkeep" or "take care" of something so that the quality or state of it stays a certain way or does not deteriorate. For example, we may use this word to refer to the physical care of something, like houses or cars. Ex: It is important to maintain your house and make sure it is in good condition. Ex: The teacher maintains order in the classroom. To "sustain" something means to use effort keep or hold something at a specific level, otherwise the state of things may fall quickly. It implies that if something is used up and is not replaced or replenished, things will deteriorate very quickly. We might use this verb in situations related to natural resources, for example (the environment) or financial matters. Ex: If we continue to abuse the environment in this way, the world will no longer be able to sustain life. Ex: This level of spending is not sustainable.

Can I replace "drives" with "gets" here?

Unfortunately no, the whole phrase "drives me crazy" means "annoys me a lot". "Get" is not correct to use here. You can say "she makes me go crazy" instead.