Color matching for Compositing

Hello everyone

I have a question on adjusting the color match of the subject with the background. 

This guy matches the whites with the brightest areas of the background with the foreground in the curves adjustment layer. And then the blacks with the darkest in the BG with the FG. 

I have confusion with the grey color picker. Does the grey color picker select parts that are not entirely black or white?

The tutorial: 

At around 2 mins 

https://youtu.be/x5TbF8Tb9U0?si=jpGYuDKwzwghfATM