Answer:
They are bought and sold by the people.
Explanation:
In the free market, it all happens by necessity, initiative, and willingness.
If someone sees that people need a transportation service, that same someone can offer a service. And someone else, by seeing that there is a demand, might want to offer the same service by a smaller price to steal some clients. Maybe someone will make a more comfortable service and offer by an extra cost and so on.
This logic can be applied to literally any goods or services in the free market.
