I've worked in FAANG. It's a fucking mess. It's a meme that they hire great people there. Most are average to good devs. I've worked on projects that had dedicated senior architects that got scrapped years later because they were obviously badly designed from day 1.
Don't know about OSS, but I would expect it to be better written/designed.
Good software developers/architects must remember to be suspicious of using meme architecture, like microservices and meme technologies like nosql, and meme research areas like ML.
Also, re usability between unrelated projects is a fake meme (unless working in non shit languages, which is never the case). In almost all cases, readability, testability, and "refactorability", should be priority. Coupling, premature optimization, abusing OOP patterns (SingletFactoryAbstractFactoryBeanVisitor), use of compiler directives, global state, parallelization, should be avoided as much as possible. Clear APIs are also very important.
But yeah, how software is made is a good idea to think of new ways of structuring code.