The power for that FET has to come from someplace, Along with its ground. Unless that board has a separate power supply input, your FETs drawing power from the Arduino.
You have a hard time teaching because you're having to reteach over and over again. You're creating bad habits. And bad habits are hard to break. You cause problems by not instilling good habits from the start. I've worked as an automotive mechanic/AC/electronics technician for over 40 years. I have seen my share of people who have been taught bad habits. And is very hard to teach them how to break that bad habit. I find it much easier to teach someone how to do something if they never done it before, or have never had any instruction from anyone else. Whether it is how to use a torque wrench, using a lathe or milling machine, or diagnosing electrical problems. It all comes down to the same thing. Teaching the right way of doing it the first time and you don't have to reteach, Over and over again. And teaching safety is a big part of that.
Teach good habits, and you do not have to reteach the same thing over and over again.