only three born in the USA

That itself paints a decent picture though, doesn't it? 27% of Californians are foreign born, yet 91% of your ex-co-workers are foreign born. Such differences can have perfectly innocent explanations (plain old coincidence with few enough employees or for a cause of some kind), but it's usually a red flag for a business taking advantage of more vulnerable employees. Anything as "minor" as low and unfair wages, or abuse, or effectively being held hostage by their employer. I cant comment specifically on where you worked, but its not just how technical jobs in the US are either.

That said, Twitter seems fine with no systemic scandals AFAIK, decent wages, roughly expected hiring diversity, etc. But with Musk at the helm now... The guy's infamous for abusing the shit out of his employees, so optimism is low