1. She is on the car.

[She is on top of the car. She is on the top of the car. ]

2. She is in the car.
[She is inside the car to drive.]

3. She is at the car.
[She is by the car... Is my explanation right?]

4. She is on the bus.
[Is she on top of the bus or inside the bus?]

5. She is in the bus.
[Does this sentence mean she is inside the bus? When she is sitting inside, do we have to use 'on' or 'in' before 'the bus'?]

6. She is by the bus.
[She is not inside the bus. She is out of the bus. She is by the bus... Would you check my explanation?]

Yes, all your explanations are correct and clear.

It's odd about 'in' and 'on' and different kinds of vehicles.
1, 2, and 3 are correct about cars, and yet 4, 5, and 6 are also correct about buses.

There's no explanation for it, but either 'in' or 'on' can mean she is inside the bus, but 'on' the car does not mean she's inside the car! Weird, huh?

¯\_(ツ)_/¯

1. The sentence "She is on the car" typically means that she is physically on top of the car, standing or sitting on it.

2. The sentence "She is in the car" implies that she is inside the car, possibly as a passenger or driver.

3. The sentence "She is at the car" can have different interpretations depending on the context. It could mean that she is near the car, standing next to it, or it could mean that she is at a specific location where the car is parked or located.

4. The sentence "She is on the bus" usually means that she is on top of the bus, either standing or sitting on the roof. However, it can also be interpreted as her being inside the bus, especially if the context indicates she is a passenger.

5. The sentence "She is in the bus" generally means that she is inside the bus as a passenger, sitting or standing within the vehicle. When someone is sitting inside the bus, we would typically use the preposition "on" before "the bus," such as "She is sitting on the bus."

6. Your explanation is correct. The sentence "She is by the bus" suggests that she is located near or next to the bus, but not inside it.