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The point is that Google (and other search engines as well) have a very large database on their computers. And a lot of fast computers to find the search results in that database. Many of those results may be outdated, but Google's freshbot updates search results on many webpages every day. That is on web pages that are known to be updated frequently.
If you know Spanish:
[google.dirson.com...]
HTHY
The key is the inverse index and the above paper shows some ideas about how to sort document IDs in the index. All this stuff, combined with some 10,000s of servers, makes the difference.