Check again. The two sites are on different domains. In order to log into both, at some point someone needs to convey state information to a third party. Gmail

Check again. The two sites are on different domains. In order to log into both, at some point someone needs to convey state information to a third party. Gmail logins are managed through accounts.google.com, so either you have a third-party relationship with accounts.google.com when you are on youtube.com, or you have a third party relationship with youtube.com when you are on accounts.google.com. Either way, you pick up a third party cookie. You'll notice that even if you never (deliberately) went to youtube and you have 3rd party cookies turned off, a cookie for "youtube.com" will show up when you sign in. I haven't looked at it carefully, but I think they just put you through a chain of redirects when you click "log in" so that you have a first party relationship with each of the domains they want to sign you in to, and that's when they give you the cookie.