We are on a bus.
Anything deemed to have a vast floor area in a vehicle is ON, while anything deemed to have a little floor space is IN. Large indicates that you can or must walk there (to your seat, the restroom, etc.), whereas small indicates that you can't walk there comfortably (you either just stand or sit).
She's in a car.
Large floor areas are seen as platforms for walled vehicles, while smaller floor spaces are regarded as containers. Consequently, it is in a car, a small boat, a helicopter, a rocket, a hot air balloon, or a tiny plane (think of an old biplane). There should be an elevator. However, we say on a bus, on a train, on a ship, on the space shuttle, etc.
Learn more about prepositions here:
brainly.com/question/4956879
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