How to implement cross-site authorization (SSO) like Google or Habr?

Question

User identification

Good evening!
For a long time I can not understand how this method works and how to implement it.
Let's say I have two web applications (a.com, b.com) and an authorization server, the user is authorized on the site a.com, after which he goes to the site b.com and there is a problem, as I understand it, the user should have your unique id on the client side so that the server understands that it was the authorized user who went to the b.com site and you can give him a token, here the brains have already boiled. How will the server understand that it is the authorized user who has entered the site b.com to the site? If through cookies, how can I get them or give them the same for two sites during authorization?

Answer 1

We go to Google and read about how OAuth, OpenID Connect work, what is Identity Server, Resource Service, User Pool and, after that, Federation. To make it easier to understand the topic, you can open a free one like in Auth0 and see how the guys do it