There can't be a person alive who "truly understands" Windows; though made by humans (allegedly), any modern OS is going to be beyond the understanding of any individual. This is the fundamental problem of managing modern systems.
I can name two: Raymond Chen and Mark Russinovich ! I don't know if Mark is still up-to-date into the latest Windows internals now that he is the CTO of Azure but Mr Chen sure is.
I was thinking Linus for Linux, but in spite of how talented these people are, it's still hard to imagine them having a detailed grasp of the entire codebase at this point.
