How do Google servers discriminate anonymous users?

Question

First of all, I am a new user of Tor and a very naive computer user in general, so excuse me if I am too thick.

When I searched anything on Google using Tor, most times I got the 'Error 403 forbidden' and sometimes I had to type a CAPTCHA. I came here and I read this question Tor and Error 403 forbidden. I followed the first option and I haven't got any 'Error 403 forbidden' since then. I don't understand why that has worked in my case, since my problem seemed very different from the one posed in the question. I have also read Does Google know that I am using Tor Browser?, but I still don't understand how it worked the answer in the first question for me.

So this is a two-fold question: Why does the Error 403 appear? And how did what I did worked?

Answer 1

Error 403 and CAPTCHA are appearing because the exit node you're using is not dedicated to you only: Many people are using it, including to access Google and other services.

It seems - on a website server's side - that one single IP address (the exit node) generates too many requests per time and a different requests/user sessions, so it can interpret it as a bot - and that's where the CAPTCHA comes out. But sometimes people do abuse Tor and use it for bot-mass-querying, and then it's reasonable to put an error 403. That's it!