Do Urban Gangs All Have Their Own "Thing," Or Is This Just A TV/Movie Trope?

Based on watching movies and TV, you get the idea that criminal gangs in urban neighborhoods tend to align by ethnicity, and that they all seem to stick to a handful of activities and “products,” for lack of a better choice of words.

Black, Latino Youth: Drugs, guns, sometimes prostitution.
White Youth: Drugs, guns, right-wing hatred.
Italian Mafia: Gambling, protection, unions.
Chinese: Gambling, counterfeit currency, counterfeit and contraband consumer goods.
Armenian: Prostitution, human trafficking, sometimes guns or counterfeit currency.
Russian: Prostitution, human trafficking, sometimes weapons.

…and so on.

Is this a Hollywood invention, or do urban gangs really try to stick to their own “brands,” so to speak?

People generally tend to stick with what they know. If you don’t have anyone to teach you how to run a protection racket, you’re unlikely to try and start one if you make enough money through other activities.

The Italian mafia was opposed to drug smuggling and dealing in some cities (and under some capi), but was the largest drug supplier in other cities and under other leadership.