Microwave ovens are designed with a special type of glass called microwave-safe glass, which allows microwaves (electromagnetic radiation) to pass through while preventing the microwaves from escaping outside the oven. This glass is typically made of materials that do not interact with microwaves, like borosilicate glass or tempered glass.
The microwave door is fitted with a metal mesh or a conductive coating that acts as a barrier to the microwaves. The holes in the mesh or the specific properties of the conductive coating are designed in a way that they are small enough to block microwaves from passing through but large enough to allow visible light to pass. This way, you can see what's happening inside the microwave without exposing yourself to the potentially harmful microwaves.
As a result, the microwaves are contained within the microwave oven, and the cooking process occurs only inside the cooking cavity. The microwave energy is absorbed by the food, causing the water molecules within the food to vibrate, generating heat that cooks the food. Since the microwave door is effectively blocking the microwaves from escaping, there is no cooking beyond the plastic door, and it remains safe to observe the cooking process through the glass window.