AC Phase Control circuit

Hello!
I am working on a heater control project for school. I found some information here:
It describes perfectly what I am looking for. Unfortunately, I am having trouble understanding the circuit.
I don't get how the current comes back in this circuit from the heater. Also, do you have any idea of the pins that are used on the arduino microcontroller?

I joined a picture of the circuit with the parts that I don't get
Thank you.

I am working on a heater control project for school. I found some information here:

What school are you in that they allow you to handle high voltage? You know that this may be lethal, don't you?

I joined a picture of the circuit with the parts that I don't get

If you use the code on the referenced page, Arduino GND goes to the GND pin of the Arduino, Digital In goes to pin 9 (controls the TRIAC) and Digital Out goes to pin 2. These pin numbers are for an Arduino UNO, other types may use different pin numbers.

Neutral is the neutral line of the high voltage (usually in color blue), while Line is the phase of the high voltage connection (has many different colors, depending on the country: brown, red, black, white are the most often used). Heater is the output signal that goes to the heater, that wire that is usually connected to the phase if not electronically controlled.

Thank you for the reply. Actually I study in electrical engineering so the project requires for me to control the high voltage circuit.
I think I get how the heater part works now.
Thanks

Unfortunately, I am having trouble understanding the circuit.
I don't get how the current comes back in this circuit from the heater.

The circuit left out the completely obvious and absolutely necessary connection of AC neutral to heater neutral.

A student of electrical engineering should be able to figure that out.

As far as electrical safety is concerned in the Project Lab, isolation transformers of limited VA (1000 VA or 500 VA) are recommended, and they are being used.