I work for a FAANG, and have for a while. Maybe I have below average IQ, too, but I've met and had the pleasure of working with some of the smartest, hardest working, kindest people in my career here. Some assholes too, of course, but we're all human.
Everything is built in-house because it needs to solve problems at a scale that you've never worked at. Be humble. If the tooling is terrible, congrats! There's a bunch of impact in your future making the tooling better. And because it's a big company, it cares a lot about marginal productivity improvements like better tooling, and will reward you for it. That's pretty different than my experiences at startups that are struggling for survival.
Maybe you picked a bad team. That's a possibility, because large companies are less homogenous than startups. But that also means that there are good teams, whereas if you pick a bad startup the whole thing is bad. Sounds like you didn't do the homework you should have before choosing a team. Maybe, again, be humble and accept that you have things to learn, even if it's just how to see red flags prior to joining a team, and use what you've learned when choosing a team next time.
Good luck!