Microwaves of that era had a proper length of waveguide do the radiation entered at the top of the cavity, and a rotating antenna distributed the energy around. A turntable wasn't that important, but some had one as well.
A squirrel-cage fan used the waveguide as a duct, and powered the rotating antenna pneumatically.
They were well designed and heavy.
These days the business end of the magnetron sits on a flange, and shines through a mica window, with the fan placed nearby in the hope it will provide a bit of a draught.