Basically, they make a large list of websites using links (links) between them. The more sites your search engine knows, the better. The only problem here is to make this list useful. That is, a large list of website features does not mean that a good result is set for search, so you should be able to say what is important on each web page.
But according to the processing power of the information you have, there is no need to stop somewhere.
This does not guarantee that you will reach every URL, but basically this is the only practical way to crawl on the Internet.
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