Google code quality

In my humble opinion, it's not the worst in the world.

It’s said that we rewrite entire code base every 3-4 years. So not much?

yep, constantly refactored, so majority is good, but documentation is usually lacking.

Ten times better (my current team in Google / former team in Amazon)

Overall very solid. There is a lot of internal complexitites, inconsistencies and duplications due to a few reasons: 1) people believing they can things it "better" or faster than integration with existing solutions. 2) being competent enough that implemention of their own shit is actually faster due to constricted solution spaces 3) everybody is a strong enough engineer to design unnecessarily complex stuff that actually works. I.e there is no negative feedback for dumb design because they can make it work. This means that a lot of our code does weird shit

Pretty awesome that Google gives engineers time to maintain high quality code.

Solid and rigorous to an annoying degree. I'd not work here if I were you