How do people in this day in age become nazis/neonazies sexist or even incels when there is so much knowledge against it? Do they get anything out of being that way?

It's a slippery slope. First it's either a community they can share anything with, or it is a subject dear to them that they see people give solution to. Then, slowly, one idea at a time, they get litteraly corrupted. Ideas are imprinted through repetition, values are suggested. Then, or before, you imprint the idea that the others are lying. This is key because it seed doubt in everything, but as he is closer from this group, this group get to imprint its own ideas through repetition alone. Distance is built with relatives so that the group is the only group he has. Then if he starts to disagree, he will be kicked, sometimes also punished, and he'll be left alone, or at least he must be convinced of it. Once there radicalisation is a process that's hard to stop.

Doubt, distrust, and a group to be with are the key ingredients. Liberalism is a fertile ground for this because it promotes individualism when humans are social creatures. So it's very easy to find people in need of a social group that gives belonging. And racism makes the easiest pretense : you belong because of your blood, or because you're born here.

For sexism, it's mostly a reactionary backlash, and secondly this liberalism problem of promoting individualism to humans who seek belonging. Feminism did won, and the old way of treating women is being addressed. But it is a process, and while we know what's bad, we don't have much new examples to follow. Yet most people have been trained in the old way, so now they are at lost. It's not the first reason why they're alone, liberalism has this place, but it is far easier to blame it on women and feminism than to try to build a new society. And also, it again gives them belonging with men like them that understands them and give explanations and solutions to their problems. Not good ones, but that's not the point.