How does multisite authentication work, like here on Toster?

Question

OAuth

Went to Toaster.
I wanted to ask a question - I sent to register on another site id.tmtm.ru.
Here it is still clear - redirection, redirection, following the link.
Registered, returned to the Toaster - here it is also clear: when you first entered id.tmtm.ru, it was indicated in the URL where to return.
Attention, question!
I go to Habr using a DIRECT link.
I am immediately asked to agree with the contract-offer.
That is, Habr found out from somewhere that I was logged in on the Toaster / on id.tmtm.ru.
How did Habr find out? After all, neither Toaster nor tmtm.ru has the right to set cookies for Habr

Answer 1

Google Single Sign-On (SSO). Cross-domain cookies and so on are just implementation details. dimonchik2013 , in principle, gave the correct link with a selection of existing approaches for organizing SSO.