Because people generally consume media that reinforces their existing beliefs, it takes broad coverage with an unambiguous message to change peoples' minds. what does the textbook call this phenomenon?

I believe the answer is: Loud signal

The consumption is perpetuated by media company's effort to cater their news in a way that suitable to a certain group of people with specific political assignment. This make it a lot harder for consumers to distinguish which of the news are factual and which are opinionated