Nah. People are indeed stupid.
However when you examine these beliefs in more depth there is obviously some amount of internal logic that is used to justify these beliefs to themselves and others in the group.
No there isn’t. It looks like logic, but it isn’t. They work backwards, rationalizing conclusions they arrive to emotionally so they can retrofit arguments to justify it, and their “evidence” as you put it is always either full of holes or outright non-falsifiable. There are always glaring flaws in their reasoning but you can’t reason them out of it because the point was always to support their preconceived notions by any means necessary. Because they’re that fucking stupid. Your entire premise is wrong.
Insane amount of what?