Microwaves heat food through a process called dielectric heating. Inside a microwave oven, there is a magnetron that generates microwaves. These microwaves are electromagnetic waves with a specific frequency (around 2.45 GHz) that is absorbed by water, fats, and sugars present in food.
When you place food in a microwave oven and turn it on, the microwaves penetrate the food and interact with the water molecules within it. The microwave energy causes the water molecules to vibrate rapidly, creating heat through friction. This heat then transfers to the rest of the food, heating it throughout.
To answer your second question, microwave ovens are designed with built-in safety features to ensure that they only produce microwaves when the door is properly closed. Opening the door interrupts the power supply to the magnetron, effectively turning off the microwave radiation. So, in a sense, the opening and closing of the microwave door acts as an on/off switch for the heating process.
However, once the microwave oven is running and the food is being heated, it is not possible to instantly stop the heating process like flipping a switch. When you press the "stop" or "cancel" button on a microwave oven, it halts the generation of microwaves, but the food will continue to retain some heat due to its temperature and the heat transfer that has occurred. The residual heat will dissipate over time.
If you need to stop the heating process in a microwave oven, it's best to use the built-in controls and turn off the oven. Additionally, using microwave-safe containers and following the recommended cooking times can help ensure that your food is heated safely and evenly.