What are some key identifying markers of a "popular source"?

Popular sources may contain ads, are often written in language for the general public, are written by journalists or staff writers, and can often be found for free on Google or other basic search engines

Popular sources are often written in formal, sophisticated language, are written by experts in their field, and are supported by research.

Popular sources contain pictures, contain ads, contain citations for research, are written by experts, and are peer reviewed