This is true of websites such as google. They are good at making it umobtrusive, but commercial sites pay to have their links come up first.Not strictly accurate. Google claims, at least, that its search engine results are not affected by advertising revenue.
The contamination of their search engine results with advertising was one of the things that turned me off several years ago on Yahoo. Google, theoretically, leaves the search results unaffected and adds unobtrusive ads to the sides and top of the page. This doesn't bother me, since I don't even notice them most of the time.
Google has results above the line and results below the line. Above the line results are paid.
Aside from that, there are companies that can move your site higher in the rankings by manipulating the way google searches.
Google ranks sites by the number of links to a site. If no other site links to yours, you are dead. It is easy and inexpensive to set up web rings that boost each others rankings. I'm sure there are other techniques used to boost google ranking.