It shows not the number of shown results which is pretty much same for any query, but number of all pages that contain the keywords. It is not really feasible to rank and show millions of pages, but is possible to do that for few hundred pages. This is why search engines work fast.
It was once possible to trawl through results pages 'near indefinitely'.
How I see it is that it is like how the music search works with Google Assistant. It can do it for 90% of the songs you are likely to hear without an online connection.
For anything else it can do the search on the server.
When you search something it is not as if a billion Google servers are consulted with the results delivered accordingly. The nearest box just knows 90% of the 'answers' and you get whatever that is, rather than a 'real search'.
If you enter in a specific code, e.g. a ISBN number, then that is like the 'unusual song' that needs actual looking up.
However it works, it is a good hustle. Actually nearer to 'the hitch hikers guide to the galaxy' in having a compiled book of answers.
