How, when logged in through, say, Facebook, do sites understand that they are already logged into Facebook, and immediately let the user through?

Question

Facebook

I asked myself an interesting question. Let's say I want to login to the Codecademy site. And before that, I logged into Facebook on my computer, sat there, and, without leaving Faceboook, closed the tab. The Codecademy website, among other things, offers to log in with Facebook. When I click, the site immediately skips me, without showing the Facebook login window and without prompting me to enter a username and password.
How does it work from a technical point of view? Or is the fact of logging into Facebook recorded somewhere in the browser and sites can somehow understand this? Maybe the question will seem primitive to someone, but I still don’t really understand this matter, and not only me. I will be grateful for the answer!

Answer 1

Cookies + oath2