When your phone nears the device, the NFC tag reader check to see if you are an authorized entrant. If so, the Arduino in the device powers a servo motor to push an arm against the lock and let you in.

Similarly, when you activate the device through the app by pushing the button, the NodeMCU Wifi Module in the circuitry detects a signal of "1" vs "0" and signals the Arduino. The Arduino proceeds to follow the same pathway as before and unlock your door.