They aren’t connected to the internet, or they arent supposed to be, I cant answer for everything and everyone in the US.
Systems running critical infrastructure are air-gapped. And protected beyond this, which I cant go into details about. But this is old news.
There are ways around this, look up Stuxnet (really, do look it up, its a fascinating story). This is a game thats been going on in re air-gapped systems for a long time.
Well, that’s sort of encouraging, I suppose. Not familiar with the intricacies of Stuxnet. Might have to look into that.