How do industry giants check if an employee is NDA compliant?

Question

Copyright

Such as google, amazon, ebay, valve, blizzard, activision and other giants.
What preventive methods do they use?
What prevents an employee from posting code on git hub or on the dark web?

Answer 1

The security service of the company interferes with him.
Your question is twofold. How they check is one thing. Usually on logs in the first place.
And "what hinders" - it is the signature put under the NDA that hinders.