Having 3rd party (third party) cookies disabled, how is it possible that youtube infers my identity from me being logged in to gmail?

• Having third-party (3rd party) cookies disabled
• remove all cookies
• login to gmail.com

• visit youtube.com without logging in

  and wonder how is it that youtube.com knows your gmail identity ??

Can anyone explain how this is achieved technically and what is the point then of disabling 3rd party cookies??