Results lists vary by search service. The vast majority of effort required to create keyword index document collections is accomplished by automated processes. A piece of software reaches out across the network and retrieves as many Web pages as it can find and follows links embedded in those pages to find other Web pages. Then the software searches the document for links pointing to other documents that haven't been indexed yet. Eventually many documents on the Web are indexed.
Spiders start with a list of a dozen or so known sites.
They index these pages, then follow every link to every new page these sites contain and index those. The process repeats until the spider can't find any links on the Web that it hasn't visited. Back when the Web was young, when it contained only a few thousand pages, this procedure took less than a day. Now it takes even the quickest spiders over a week to roam the entire Web. The database grows in size because the indexing software saves new links it discovers, as well as user suggested URLs, which can be explored and then indexed in the next cycle.