Code behind the Code

Since I have been a member of this group, I have tripped over "G"
Code . I have asked people who are considered experts in CNC
programming,"What is the code behind the code?". They haven't got a
clue as to what is going on behind the code. They just use it because
it works.

Where can I find a source for the code behind the code? What is
really being processed in the conputer when the command G03 is being
executed? Any body besides me interested?
Is there anybody in this group that knows and would share that
knowledge?

Thanks a million well in advance.
Wally6800