What you see in the video from above is the Stirling Engine, a simple engine in which hot air moves a piston in ways you never knew possible. As you see, the piston is smaller than the cylinder it’s in. When the lower metal place is heated, the air that is at the bottom will heat up and then push the piston while going up.
Then the cool air from above the piston will be pushed down and heat up. The cycle begins again. While such an engine isn’t extremely practical, it serves quite well as an example of what engines are: devices that transform energy into mechanical work. That’s all there is to know about engines. Way for the Stirling Engine.