I thought the quality was pretty high, largely because there were a lot of rails constraining how code should be written. Most of the code I dealt with was written using somewhat rigid (but generally well-designed) frameworks with programmatically-enforced style guides.
Also, most work seemed to involve some balance of junior and more experienced people, which helped keep quality higher. Outside of Google, I've seen pretty large projects written by new grads with little supervision (and on a tight timeline). Those codebases can be pretty hairy.
The thing that impressed me most about Google was the encoding-of-cultural-norms-in-various-CI-jobs.
It lets them extract usable SWE horsepower from pretty much anyone who steps inside and at least tries to be useful and not just coast. They can ingest a startup engineer, someone who's been a mid-tier enterprise codemonkey, yr mythical 10xer, the whole statistical gamut.
That honestly does seem like a recipe for good code. And sure, there's tons of open source out there of dubious quality.
@resource0x in a sibling comment made the point that it's possible to write great code even if the program is a flawed design. I'm probably conflating those things.
Also, most work seemed to involve some balance of junior and more experienced people, which helped keep quality higher. Outside of Google, I've seen pretty large projects written by new grads with little supervision (and on a tight timeline). Those codebases can be pretty hairy.