From the beginning Google has used a "PageRank Algorithm" to help ensure that quality sources are ranked higher up in your list of results. What this means is that Google prioritizes results not just with your keywords in the title or mentioned a lot, BUT also results that have many quality pages that link back to your resulting page. Over time Google has tweaked their algorithm to consider a searching person's geographical location and previous searches.

Google hires "search quality raters" to search on topics and then evaluate the results based on "E-A-T" which is Expertise, Authoritativeness and Trustworthiness. But, the most important consideration for ranking a site as more reputable is still that many quality websites are linking to that website - which is possible thanks to the "PageRank Algorithm." (How Google delivers reliable information in Search) 

"How Google Works" by Mark Smiciklas CC BY-NC 2.0