How is the registration of users on the site from a technical point of view?

Question

User identification

Interested in things like:

• What happens to the completed registration form after clicking the "Register" button
  
• What happens to the entered data in the "Login" and "Password" fields after pressing the "Login" button
  
• If the site has a user rating system, how does the server understand that the "thumbs up" button will be assigned to this particular user, and not to another?
  

Answer 1

1) sent to the server, for example, via a POST request
2) the same, sent to the server. Perhaps in a processed form (a hash of the password instead of the password itself). In response to a login request, session cookies may be sent, by which the server will automatically identify the user without the need to enter a password.
3) cookies that the browser sends with each http request - they may also contain the username. Yes, and the session ID, which I already mentioned.
The maximum size of cookies depends on the browser, but, for example, 4 KB of cookies in EACH HTTP request is quite realistic. Considering that several dozen requests are sent when loading one page, I consider http a very inefficient thing.