Here is a simplified diagram.
Power is applied (by the transistor) if:
- The switch is closed OR
- The Arduino sends a signal to keep power on
I didn't represent the rest of the circuit as I believe this is not relevant.
About the Transistor I used, I didn't know this difference between IRF and IRL. Unfortunately mine seems to be IRF (although I'm not sure about what it means). Would my setup work with an IRL MOSFET ?