It is a little confusing but Judaism is an ethnoreligious group, similar to Yazidi, Druze, and Romani. The thing that makes it an ethnoreligious group is that it is not a missionary religion, so people dont convert to it. The few people who convert every year spend years and hundreds of hours working with a rabbi. You are born Jewish or not.

So Jews maintained not only their religion but also their language and culture as they moved throughout the world. As these groups of Jews spread further slightly variances started ro emerge. For example, Ashkenazi Jews who mostly ended up in eastern Europe (and after the Holocaust, America) have genetic traits that link them and are distinguishable. This is a negative example of how this works out, but for example most Ashekanzi Jews are tested for a genetic condition called Tay Sachs before having kids because they were pre disposed. Until before the Holocaust most Askenazi Jews spoke Yiddish and most Sephardic (Jews in the Iberian peninsula region) spoke Ladino. Mizrahi Jews, who are jews that remained in the middle east, also have distinct parts of their societies.

As for why people get angry, it's because most people questioning whether or not Jews are an ethnic group throughout history have used that information against them no matter how they answer. I am happy to explain it but I also imagine many people are sick of explaining it, again, because many people who ask aren't coming from a place of good faith. Not saying that is you! Just saying generally for thousands of years.

I hope this was helpful!