Comparing the Arduino Uno and the Nano, you'll notice that the former includes an onboard DC barrel ... to the supply's positive wire and your Nano's GND pin to the supply's negative wire.

The device he holds in his hand is an Arduino connected to the OUTSIDE ... Apparently the DC barrel jack on the outside of the lock serves as a one-wire protocol interface. Once communications ...

Comparing the Arduino Uno and the Nano, you'll notice that the former includes an onboard DC barrel ... to the supply's positive wire and your Nano's GND pin to the supply's negative wire.