The way that Aluminum Foil stops car thieves is the Aluminum foil blocks out signals that are sent and received from the Key Fob which car thieves use to hack and I believe create another key fob to work in your vehicle to steal your vehicle.
Your Key Fob has to be near the car for the doors to unlock and for the vehicle to start so the aluminum foil does the job of blocking out the signals to the hacker.
I'm no computer expert so I don't know 100 % how they actually hack the signals but that is what I've read online.