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.

Even the physical LCD is still active with the backlight off. On older laptops, you can shine a light (i.e. phone flashlight) through the apple on the back of the display and see a spot of the display, but the new ones are lame and don't have a glowing apple*.
_{* Sarcasm... I think}