They don't distribute it to contractors, I am pretty sure. If contractors work on it they are under NDA, with no rights to use the code for any other purpose, s

They don't distribute it to contractors, I am pretty sure. If contractors work on it they are under NDA, with no rights to use the code for any other purpose, so considered inside Google. If they weren't you would find many copies on github. Edit: I have no idea whether Google has contractors in such areas. I worked for a company of equal size many years ago and we had contractors everywhere. It was a massive bureaucracy to get access rights for what they needed to do their work, but in the end there was probably not much in the company that some contractor had not access to.