Given how large those systems are, how much layers (OS) of code there are, and the turn-over in software companies, I am pretty confident that no single engineer know everything about.

"engineers" - it's been decades since anyone on the planet could know everything about a whole system, but each engineer knows everything about their subsystem

Which is why I won't fly on certain airlines :-). As a Software Engineer who worked at an avionics company, I know who our big customers were and what bugs are in the products they bought, so I avoid them whenever possible.

most hardware and plenty of software for that hardware is sourced all around the world...engineers involved make assumptions based on how that stuff should behave, perhaps even run it through tests of their own...but they hardly know _everything about it_