Question on spark gap transmitter circuit
That circuit works just like old door buzzers worked -- it's just a simple relaxation oscillator.
Any oscillating electric circuit creates electromagnetic waves. The buzzer creates EM waves on a large frequency spectrum which is why it can be picked up by the radio.
There are two problems with using this circuit for communication:

Radio transmitters today are required to transmit on a very narrow frequency band which this device doesn't and therefore it wouldn't be approved by the FCC (or your local regulatory body).

The reception range is very short. If you went to the next room you probably wouldn't pick it up.