Most are unique in the way they procure and organize information. It would take a book to describe how they manage to digest millions of Web pages and determine which pages are most relevant to the search words entered by the user. For those interested, however, searches of the Web itself on how search engines work brings up loads of information. The amazing thing about the World Wide Web is that just about anything you can think of can be found once you've learned how to manipulate search engines. Advertising often pays.
Universities also sponsor search engines.
Government agencies also have search engines. It lists major search engines, how to use them and how they work. It looks for search engines that excel in specific topics such as general, adult, automotive, aviation, dining, etc. and can even look for search engines within a given country. This is my favorite search engine. I like it because it employs five search engines to make a parallel search of the keywords or phrases entered.