You are on a bus when you think of it as transport.
- I am on a bus with my dogs. We will be with you soon.
(The bus as transport.)
- By next week I will be on my boat, relaxing with a cocktail.
You are in a bus when you think about the inside of it.
- I left my bag in the bus.
(The bag is inside the bus.)
- In the boat there are three cabins.
If the transport you are talking about has only four wheels, a car for example, don’t use on. Only use in.
I am on a taxi.
- I am in a taxi.
- I left my bag in the car.