The operation of a microwave oven is really very simple. It consists of two parts: the controller and the microwave generator.
A schematic diagram of the microwave generating circuitry and portions of the controller is usually glued to the inside of the cover.
The controller is what times the cooking by turning the microwave energy on and off. Power level is determined by the ratio of on time to off time in a 10-30 second cycle.
The microwave generator takes AC line power. steps it up to a high voltage, and applies this to a special type of vacuum tube called a magnetron – little changed from its invention during World War II (for Radar).