I have experienced abuse at the hands of supervisors (generally disrespectful / belittling behavior). You have to have a special kind of privilege to have not experienced this sort of thing. I haven't ever experienced such a pervasive environment of targeted abuse as in the OP, but it certainly sounds believable. It's the reason why unions came into being. We didn't go from being a society of slaveowners to a society of saints in the past 150 years.
I have learned hard lessons though:
HR is not there to help you. I consulted HR and a senior director about my previous supervisor's toxic behavior; not only did they do nothing, but they said they had previous complaints about them! And of course, I knew this, because I had many friends in the org, and I had witnessed the disrespect this person displayed towards others.
Collect evidence and get a lawyer if you want to stay in the org. This is the only way to protect yourself - it becomes very difficult for them to fire you in retaliation. I haven't taken this route, but I've seen others do it.
My advice would be to make plans to leave. OP was in a tough spot due to citizenship. But you have to leave toxic environments. You will not fix them, and if you could, you'd be better off quitting and starting a consultancy to fix those places. Find someplace new and be more particular about joining them.