In final form, a Bluetooth module and Arduino Pro Micro are stuffed inside the stereo along with a DPDT relay to switch the I2C bus.

The DPDT relay (Double Pole Double Throw), also knows as the “2 pole relay”, is quite interesting and can be used in various scenarios, including for changing the direction of a motor as you can see ...

Using custom PCBs hosting banks of DPDT relays, he progresses from the basic AND and XOR gates to half adders and full adders, explaining how carry in and carry out works.