most microwave ovens do contain a metal plate or a metal grid called a "waveguide cover" inside the cooking chamber. The waveguide cover's primary purpose is to prevent food particles and splatters from entering the microwave's magnetron and causing sparks, which could potentially damage the microwave or even create a fire hazard.
The microwave's magnetron generates the microwaves that cook the food, and it is a critical and sensitive component. If any conductive material, like a small piece of metal, were to enter the magnetron, it could cause electrical arcing (sparks) and potentially damage the magnetron or other parts of the microwave.
The waveguide cover is made of metal because metal is non-permeable to microwaves. It serves as a protective barrier that allows the microwaves to pass through to cook the food while preventing stray particles or splatters from reaching the magnetron.
However, it's essential to follow the manufacturer's instructions and guidelines for using the microwave oven to ensure safety. For example, using microwave-safe containers and avoiding putting items with metal, such as aluminum foil or metal utensils, inside the microwave, as these can also cause sparks and potentially damage the appliance.