Most of the engineers at those companies (and really all companies) don't work on those projects. Some people work on truly complex projects, but those people are a tiny fraction of the entire workforce. It wouldn't even make sense for a company to allocate people that way.

Also, I'd contest the statement that Google or Facebook works on the most complex software. They don't work in fintech, medical, hard real time that I know of (waymo does, but they've been spun out), and many many more fields of SW, HW and CS.

They work on hard stuff, but don't discount the complexity that other companies deal with.