Magnetron tube assembly consists cylindrical cathode, a heating filament to heat the cathode, a metal anode, and an antenna. The heating filament, which is present in the center, is attached to the cathode, and the cathode is enclosed in the anode cylinder.
When filament heated the cathode plate, it emitted electrons. These electrons are controlled by magnetic fields. Controlled electrons are responsible to generate high-frequency microwaves. You can read magnetron tube working at this link.