How illegal it is to read Windows source code?

You are treading on thin water now. Windows devotes thousands of people to stop all attempts of spamming and stealing valuable information.
One consideration is to decide if you would enjoy being labelled as a thief for the rest of your life.
No job will employ a devoted tech with an idea to cheat others.
I can not say if the CIA visits you. Just know there are vast things to explore with gaming and enhancing others abilities and make fabulous money.
Windows ten does have insiders insight program, often contacted to evaluate, but we find bugs others plant. Please advise if this is helpful.