A lot of people are confused about why we "get on the bus" and "get in the car". How is it possible to be on something when you are actually inside?
We shouldn't take the expression of being "on the bus" literally. This phrase was taken from "get on, " meaning to board, embark, or mount (something). So, we would actually be getting on board when we enter the bus. Instead of saying "We are on board the bus," we say "We are on the bus." In the same way, if we were to "get off the bus," we would be disembarking from the vehicle.
This is also applicable in the context of trains, aircraft, and ships.
On the other hand, you simply and literally "get in" or "get out of" a car.
