[hnbad]

They don't care about genetics. They just dislike specific groups of people. Which ones is largely arbitrary and has more to do with history than genetics. Some places also have a much narrower "in-group" than others, again largely as a result of history. In some cases the "in-group" doesn't even include all the groups of people that historically live in the same territory, e.g. present-day China has significant undertones of Han nationalism or India with Hindu nationalism, despite both countries being geographically huge and ethnically diverse.

In the moment, aside from historical preconditions, the largest factor to how narrow this in-group is and how strong the dislike towards those not in it is, is mostly a matter of perceived scarcity and danger. As every good business owner knows, the best way to distract an employee from how small their share of the profits is, is to dangle a more desperate group in front of them as a scapegoat. Give someone your scraps, then make them deathly afraid others want to take it away from them, and they won't dare to ask for more (or solidarize with others against you - racism was one of the biggest cudgels against unions in the early 20th century US before simply framing unions as communist became an option).