how does a theater popcorn machine work?

There is a very hot kettle with a fan-like arm that rotates inside the pot, thus stirring the contents. Pour the popcorn kernels into the pot, then pour in oil and any spices such as salt. The heat in the pot will warm the oil, which will fry the popcorn. As the popcorn begins to pop, it will be projected into several different directions as well as expand in volume (puffed up with air). Popping kernels that fly up will hit the top lid of the pot, forcing it open, and fall into the cabinet. The others will inevitably fill up the kettle, as each popped kernel has a significantly greater volume than an unpopped kernel, and the stirring motion will eventually cause them to overflow out of the kettle and into the cabinet.
When you don’t hear any more kernels popping, turn off the heat element and stirrer in the kettle, and empty the remaining contents of the kettle into the cabinet. Waiting too long can result in burnt popcorn, and in extreme cases, fire.
Whole popcorn machines are sold in some retail outlets and online. But if you’re handy enough to build a kettle with a heating element and a stirrer, and a cabinet for the popcorn, then more power to you!