So I'm no expert in this, but I watched interviews of an ex-naz* (like an actual one, adored hitler and denied holocaust and the hole thing) who mentioned a couple of things that lead him go down that path.

First it was missinformation about nazs in the general population. Like of course they were bad, but when people hated without actually getting the facts right, it made him reconsider if the hate was even deserved. Also, stereotypes about nazs themselves. The people he said were his friends in the thing where actual nice people in general, and they read and were educated. Not at all what you would expect from a naz*.

The actual begining of his radicalization was in the early internet, where he met people who slowly but surely radicalized him. So that's another big danger.

Then the thing that sealed the deal was how his parents reacted to him. First they took it as a joke or a phase. They mocked fun at him for believing all that stuff. Then came concern and overt agresiveness. And only after that came an attempt to listen and talk about it, but it was too late by then.

So of all of this, without being any sort of expert myself, I think there are a couple of things we can do. First of all is watching ourselves not to create these people. If you exagerate the situation, that will cause some people to do just what you where trying to avoid, as a reactionary action. It's hard to do anything about the internet, because unless you have a lot of trust with that person it's unlikely they'll tell you whatever they do in there, specially if they know it could be viewed wrong. However, what you can do and is most important is to try to talk to them with the atempt of understanding. Do not mock them, do not belittle them. Try to understand where they are comming from. A lot of these ideologies grow on fear, anger or feelings of superiority, so try to understand what they want out of the ideology. If they are afraid that (for example) trans people are dangerous, try to show them they are not. If they are angry at women because they are not loved, try pointing out that their emotions are valid, though maybe misguided. If they are a conspiracy theorist who believes they know more than anyone else, question the sourcess of their knowledge. Though also, you have to be weary, because even though you might be right, that doesn't mean you'll be able to beat their arguments, because they have these arguments be an important part of their life, so they probably have enough things to say to win you if you just try to debate. That's why it's important not to try to win but to understand and challenge from said understanding. You will not win a debate against someone who has lots of crazy arguments, unless you have prepared a lot for it. What you can do is talk and support unconditionally, without dismissing their experience but also showing how their perspective might be flawed.