The entire feed is conspiracy theory videos which have led them to believe some really awful things. Virtually any tragedy in the news has some conspiratorial take; the victims are actors, it was a false flag operation, the school doesn't exist. Over time the viewer builds up the impression that every news story is meant to deceive them, and then completely loses their ability to judge truthfulness or apply skepticism.
I've even heard them talk about leaving their idyllic home because of misplaced fears. For the rest of their lives they will believe the awful lies that YouTube has served them.