Some include summaries of the web pages included in them. Because they are more selective, they are a good place to start a general search or survey what's available on a broad topic. Another way to search is to use Search Engines that allow you to enter search terms. Relevant web pages are matched against those terms and displayed on screen. Start here when your search is narrow or well defined.
Nothing covers the whole of the Internet.
If a search fails to turn up useful resources, turn to a Meta-search Engine to gather resources from all the major search engines simultaneously. Have a set of relevant keywords ready. Know what type of information you want and what you want it for. Use the most specific search terms possible. Once you have found a web site you need to know if it is any good.