It's called radicalization. It usually involves taking a vulnerable person who has beliefs aligned with the group and slowly encouraging them to believe more extreme things. Eventually they get them to a point where they see their opponents as an existential threat. That's when the violence happens
Suburban kids are susceptible to the fascists/nihilists because they're usually in a conservative environment concerned about loss of class or privilege. They believe change is bad and "Others" are a threat. A lack of in-person community makes it's easy for recruiters to isolate them mentally.
Yes. Terrorgram and rightwing ideological grooming is a form of radicalization.
I'm saying the term is radicalization. We have specific terminology to refer to this phenomenon. We should use it, especially because we only see "radicalization" used in reference to Islamic terrorist groups. But it's the same process that the far right American groups use.