They see things that confirm in their minds the biases they already have, without realizing they were biased. They already have a bunch of ideological beliefs but they have no awareness of ideology, so they think there beliefs are real and natural. Someone comes along selling propaganda, and if the propaganda says when they already believed they think "Hey this guy must be telling the truth" and from there they'll start to believe other things that the propagandist says and eventually you end up watching OAN and nodding along.