US.

No comment on the rest / some of it I honestly don't know.

None of the organizations involved share a lot of data / answer questions / you don't ask, and everything is designed around you not knowing.

I'm not the original commenter but I'm familiar with policies like this. I grew up in Lynchburg, VA, where the Naval Nuclear Fuel facility is, operated by what was B&W. It had identical policies: everything that goes in, stays in, no one allowed in without clearance, no paper or electronics leave, air gapped computer network, etc. Not the monitored bathroom breaks, though....

I'm guessing Cavalier, Mountain Home, or Tonopah.

Given the intensity of the security described, I strongly suspect the answer to each of those questions is "I can't say".

We outsource pretty much anything to contractors, though.

...including security clearance investigations...

Sure but never bad to ask :)