How Do GMs See Whole & Parts of Board at Once?

This feels impossible to me.  But it seems that some GMs (unless my perception is wrong) can see the entire board at once and know where everything is, while simultaneously calculuating and evaluating specific parts of it.  

How is this possible?  

I have to consciously make the decision to shift my perspective between part and whole everytime I'm looking at the board.   

Any tips on how to do both at once?

I am not a GM. I just play for fun. Study the basics and from there you develop your own technics. Thanks

Playing for 20 years and 20,000 hours seems to be one of the keys here.

I don't know.  What about those 15 year old GMs?  And what if I find myself still not able to improve after many, many games?

Well, ok, 12 years and 12,000 hours.

If you can't improve after many games then you need to change your learning strategy.