How can different Google sites share cookies [duplicate]

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• Cross Domain Authentication 2 answers

(I don't know if this is the right SE site for my question; if not, please direct me to the right site. Thanks!)

I'm learning about the same-origin policy and begin wondering how different Google sites (with different origins) can share the knowledge that a certain user is logged in. For example, logging in on https://www.google.com will also log me in for https://www.youtube.com. How can the latter knows?