Electromagnetic cavity resonators are used in?

UHF/SHF circuits, mostly electron tube circuits, frequencies from around 1 GHz (1000 MHz) to 30+ GHz.

Although it's possible to use cavity resonators at lower frequencies, they become very large, and their theoretical efficiencies are not so much better than conventional circuits that the cost/size can be justified.

Electron tube devices are being replaced by solid-state devices for all but very high powers, such as radar, deep space tracking and nuclear research.