I'll answer your ridiculous questions, though I suspect you'll simply continue to deny reality.
And where would this assumption you claim exists — without any citations, BTW — have come from?
You're asking where stereotypes come from? That's certainly not so simple. Gangs used to be predominately white, after all. Not just any white people, of course, but white immigrants. Immigrants who suffered from discrimination and abuse simply because of their national origin. Remember "No Irish need apply"? Many researches believe gangs are mostly an immigrant problem and that long-standing black and latino gangs are a symptom of ongoing racism. (It's a lot harder to tell that a second generation Irish teen is Irish than it is to tell a 10th generation hispanic teen is hispanic.)
Why do they form? That's pretty simple. When you're pushed out of the economy (as is the case for white immigrant gangs, and well as other gangs today) underground economies are created out of necessity. When you can expect not protection, but abuse, from the police, it's only natural that alternative forms of protection come in to being. Gangs meet these needs rather well, providing resources and protection to their communities.
Allow these disadvantaged communities access to the larger economy and offer real protection by police and government and you'll see the need for gangs disappear, just like Irish and Italian communities before them.
Until you can explain, why the same "historical inertia and ongoing racism" — whether they actually even exist or not — do not cause Asians
There are dramatically different stereotypes associated with Asians -- most of them exemplifying positive traits. The most popular? They're good at math, they work relentlessly, and they're terrible drivers. Consequently, they're not as subject to things like police harassment as black and hispanic people, who are considered, by default, to be criminals because of absurd racial stereotypes. They're expected to perform well at many work-related tasks simply because they're Asian, where black and hispanic people are assumed to be lazy.
That doesn't mean those positive stereotypes aren't harmful -- all stereotypes are harmful. The point is that they face fewer challenges that black and hispanic people when it comes to issues that lead to financial success.
Where did those stereotypes come from? Our perception of Asian schools and parents. We think that Asian schools are strict and disciplined, almost to the point of being abusive. We hold the same perception about Asian parents. I suspect this developed sometime after WWII when we started to take a greater interest in Asian cultures. Why math specifically? I'd put that on Korean immigrant children, who study math differently than we do here in, putting Korean immigrant students just a little bit ahead when they first arrive.
historical inertia and ongoing racism
Stereotypes are a form of racism. As for inertia, I'll blame confirmation bias. Not just in the case of Asians, but for other groups as well.
Please, explain, how a "racist cop" manage to distinguish between Black immigrant and Black native born.
That's laughably simple. Just listen to them.
Not that the cops are likely to do that. We get a new story every week of police abuse and unequal treatment of white vs black suspects. Mass shooter evades police before being caught? He's white, so they go out and buy him food from Burger King because he's a little hungry. Selling loose cigarettes on the corner? He's black, so choke him to death. Pregnant women dropping her kid off at school has a disagreement about parking with a fine upstanding white woman? She's black, so slam her face-down on the ground. All blacks are violent criminals in the minds of police.
Police have a long history of treating people of different races and national origins differently. You'll find that we'll see a dramatic change in black and latino stereotypes once police abuse becomes the exception, rather than the rule.