If the brightness of the built-in display is turned black, can it be considered off? Model: MacBook Pro (Retina, 13-inch, Mid 2014) OS: 10.13.6 

I connect an external monitor to the macbook, set the Arrangement to be Mirror Displays and then use the keyboard F1 to dim the built-in display to black. It seems it's off and even the back apple log is black (since it's using the same light as the monitor). 

So, can I consider the built-in display it's off? (means it will not use any hardware/software resources such as CPU/RAM etc..). Thanks.
The display is still using resources. You can prove this by opening the Digital Colour Meter. Just hover your mouse to the built-in display which is currently dark. You will still see the content in the Digital Colour Meter on your second screen.