You need a power source, a way to level convert the driver pins, and some protection in case something goes wrong with the circuit ... he was able to control the relay using 3.3V, 5V, or 12V.

They are known as components and each one is represented by its own symbol when drawing an electrical circuit diagram. The flow of electricity is invisible and it can be difficult to understand ...

So the additional relay allows a circuit that’s independent of the coherer — a sort of mechanical amplifier. Notice that the diagram shows the use of U-shaped electromagnets for the relays.