The way I was taught/remember, in this example: if you are an 8-Ohm device, “looking” at your terminal connections on the patchbridge, you will see 8-Ohms as your closed loop.
If you are the 100-Ohm intercom, looking back at the output terminals, you see 100-Ohms in THAT loop.
Easy in the fixed-volume bridge.
For the variable, this “looking back” impedance could vary from 50-100. It would mean at zero resistance in the variable resistor (max volume), you’d have only 50-Ohms total in the bridge...might lead to some distortion, due to impedance mismatch.