The Web is huge - 800 million pages by recent estimates Search engine searches are NOT actually searching the web . They are searching proprietary databases compiled from samples of information found in web files.
Search engines do have some overlap in data.
Most of these methods are well guarded 'trade secrets' and must be discerned from Help pages or trial and error. Search techniques may not work the same way or produce the same results across different engines so they can't be relied upon without experimentation. No standard vocabulary or subject classification is routinely employed for web files. The web is continually in flux yet search engines may take between 2 and 6 months to add new pages or remove expired ones. Search engines are also in constant flux.