It is achieved by a process called Convection.
Let's say for liquid in kettle, heat is applied at the base of the kettle. The water at the base becomes hot and also less dense. So it floats up to the top of the kettle. The cooler water at the top of the kettle which is more dense falls to the base of the kettle. And this is also heated and it also becomes less dense and it also moves to the top. So heat flow is achieved by the cycle of mass motion of the heated fluid The cycle continues.
This is a simple convection cycle. It applies to air and gas as well.
