My info is dated, but generally the recorders were down in a telecom room below/behind the main portion of the facility. They were being converted to solid state stuff back then.
Whether or not the controller in the cab or radar facility back then had one frequency or ten frequencies selected at their station (they usually had the ability to both select almost anything to listen to and then also separate switches for transmit, besides the hoot and holler phone lines to various other facilities) didn't really make any difference in what got recorded where. The recorders were a one to one ratio with the transmit/receive loop.
Nowadays if they've come into the new century, it would be likely that it's all a bit "smarter" but I doubt much VOIP has been implemented anywhere but the transport of the circuts. The real challenge back then was every critical circuit needed a backup circuit that didn't take the same physical path from the facility to whereever the transmitter was located, since they're of course, often hundreds of miles away. Getting a redundant loop to a hut in the middle of nowhere was a pain in the ass. Usually the hard ones were done with a combination of wireline and wireless microwave fixed shots.
But that was all "outside plant" stuff. My company's gadgetry was used at the TRACONs and Centers for the conferencing of various back channel facilities together -- stuff like when all the NYC airports decide it's time to flip all the runways for a wind shift... They'd all jump on our box via a touchscreen thing that would conference all the supervisors together nearly instantly, so they could coordinate.