I have been seeking an intercom unit for a Cessna 172N that has four seats, but only two headset jacks. I would like rear passengers to be able to easily communicate with the others.

Aviation intercoms have external transmit switches, but so does the airplane. How does the transmit function work in this regard? Do they send a certain audio signal frequency to the COM stack that tells it to transmit?

Are most intercoms universally compatible with all com stacks?