Here’s how I understand the issue:
A keyfob is a radio Transmitter. To unlock your car you need the radio transmission to reach the car. The keyfob doesn’t transmit a signal when at rest. Therefore putting a keyfob in a Faraday bag achieves nothing.
Correct. So, I build a receiver to pick up the signal from your fob, and then I re-transmit that same signal to your car.
It’s slightly more complicated than that. But not by as much as you’d think.