Here are the internals of a cheap Microwave/Convection Oven combo. Electronics bay is pretty much the same as a standard microwave, with the magnetron, transformer & diode/capacitor voltage doubler, with the addition of an extra fan & a pair of nichrome elements to provide the convection oven function.
Convection blower which keeps the cooking vapours & smoke away from the elements, & circulates the hot air around the cooking chamber. This is a 12v DC centrifugal type blower.
The elements are inside this steel shield, air duct extends from the centre.
This oven has a pair of thermal switches on the magnetron.
The usual capacitor/diode voltage doubler in the magnetron power supply. The transformer is visible to the left.
Electronic controller PCB. This has a pair of relays that switch the elements & the magnetron transformer.