> I keep hearing this theory, but it just begs the question: how are these people deciding which sources to trust? What makes them trust Fox News, Marjorie Taylor Greene, or Alex Jones rather than other "mainstream" news?

Abstractly, the same way ~everyone does: heuristics, confirmation bias, post-hoc rationalization, etc. On a relative scale, some people's methodologies are better than others (often much better), but on an absolute scale they are almost all bad.