I think if you saw it on a bench hooked up to wires, it would be intuitive that it was a circuit. It's equivalent, but instead of being coupled via wires it's coupled via RF. I think it feels like there's no return path and that the circuit is open, but it's a real circuit with complicated/uncommon coupling to the power source.

A resonator is both a component in the circuit (the case is a cavity resonator) and the type of circuit this is. When illuminated (or hooked up to a power supply on the bench), it produces a sine wave, and holding all else equal the frequency is a function of the capacitance of our membrane capacitor. That membrane is flapping about due to sound, changing the distance between the plates of our capacitor and thus it's capacitance. So this shifts the frequency we're resonating at and encodes the audio into our output signal (frequency modulation).

So it's very similar to a standard LC resonator circuit you might make on a breadboard.

I'll leave you with another story of clever KGB sabotage. The KGB controlled facilities used to construct the US embassy in Moscow in 1979. They were able to extensively bug the building. They were also able to mix thousands of diodes into the concrete. This defeated NLJD (https://en.wikipedia.org/wiki/Nonlinear_junction_detector) based bug detection because they detected the diodes in every direction.