So let's say you're logged into google's services using Firefox. You visit a page and it downloads a bunch of google owned JavaScript and a few fonts and image

So let's say you're logged into google's services using Firefox. You visit a page and it downloads a bunch of google owned JavaScript and a few fonts and image assets. How do they connect it back to your account? Does that JavaScript have access to the google cookie, or does that get blocked by cross-origin restrictions? If it's blocked, then what are some other mechanisms to connect you back to your google account?