Windows source isn't all that hard to see. I know they've made it available to some universities and large customers also can get access.
https://www.microsoft.com/en-us/sharedsource/enterprise-sour...
https://www.microsoft.com/en-us/sharedsource/enterprise-sour...
And, IIRC, infamously the Chinese government too, because they made it a pre-condition of them purchasing Microsoft licenses that they must have source code visibility.
Well, there is for sure a lot to criticize about the CN government, but this precondition seems to me very natural (the OS is a very natural place for possible backdoors, otherwise...)
Even the communist party in China is more up to date then my own government