With the application and systemd service configured and installed on the target SBC, it will automatically start and configure itself. To complete the system, you could install it along with a suitable power supply into an enclosure, into which you would run the signal wires from the switches, the network cable, and so on.

One implementation of this system was installed at the Entropia hackerspace in Karlsruhe, Germany. This setup uses a real traffic light (legally obtained) outside the club door with 12 volt LED lights for status indication. The SBC, relay board, debounce board, and power supply (5V and 12V MeanWell industrial PSU) are all integrated into a single, laser-cut wooden enclosure:

However, you are free to integrate the components any way you wish. The main thing to consider here is that the electronics are all safely protected from harm and accidental contact as the relay board could be switching mains voltage, along with possibly the mains voltage line for the power supply.