You’re on the bus, in a seat, but if the bus driver finishes the day and left his hat behind, his hat is in the bus on a seat. Active/private/static vs passive/public/transitory. You’re generally in buildings but on vehicles, unless that vehicle is both private and enclosed. It’s not much more complicated than in[side] vs on [top of]; just keep in mind that it’s predicated on whether or not the encapsulatory nature of the object is necessary to its identity. For instance, you could also ride on a flat parade float without walls on roof, and putting a box on it to make it a bus doesn’t change that, so it remains ‘on’.