Oh ok i got it, the current that exits the transistor via collector pin enters the voltage supply and then go back in the transistor via emitter pin, it never go in the Arduino, this answer to my first question thank you.
By the way, should the voltage supply be the arduino, we should not use the digital pin in output mode, or we would give him too much current right?
Does the 5V supply pin of the arduino have a different max current value than the digital pins?
And is the ground pin on the left side of the arduino connected to the right side ground pin, so that they always have the same voltage?