It sounds like you landed on a shitty team and you should try to find another team, look for a mentor, or move on. There are a lot of programmers with terrible personalities, and not only are you not the first one to encounter this, but you're not rare in having this experience.
There's so much in this field that no one knows it all. I've been doing this for almost a decade and I still encounter concepts and technologies that are new to me. I've gotten deep into Haskell of late, but I still don't know what "homotopy type theory" is or much category theory beyond the superficial. That's OK. Just learn things and, if you have time, build things. You'll get better over time.
Also, the false sense of incompetence (impostor syndrome) is very common in the early years. It's depressing and brutal, but you can beat it.
[H]ow should I act in order to maintain my dignity but also be humble enough to acknowledge the co-worker's knowledge?
Find people who you are comfortable asking for help from. You have nothing to be ashamed of. No one was born knowing how Git works.
Is it normal to be hired as a junior and just being thrown to the lions, with no help or time to ramp up?
Dysfunctional but fairly common, especially in the "hot" Valley startups that are often run by adolescents. You're not alone. You'll get through it. Just keep learning, not because you feel a need to, but because you're interested. Of course, if you're not interested in learning more about programming and CS, then it's time to consider something else.
Do all star/ninja/rock-star software developers have rotten attitudes?
First, I'd bet that they aren't that good. There are plenty of people who can talk a good game, who know buzzwords and hot technologies and trends, but are mediocre programmers. They write code fast and they're great at making themselves seem smart (especially to non-technical business operators) but their code is often of low quality.
Most of the really good programmers are humble, just because it's really hard to get any good-- you have to consistently fight your way to better projects, which often gets political and that can be very draining-- and it takes a long time. Although our industry fetishizes youth, almost all of the great programmers are over 40, just because it takes so long to develop the skill.