There were blogs that explained this in detail (Facebook does something similar), but I can't find them, so here's what Google's AI overview says when I search for "How gmail cookies help google track users across the web":
Gmail cookies, such as SID and HSID, act as unique identifiers for a signed-in Google account, allowing Google to track user activity across its services and millions of third-party websites. These cookies, often lasting 2 years, link browsing behavior—like searches and site visits—to a specific user profile to personalize ads, measure campaign performance, and analyze site usage, even on non-Google sites that use tools like Google Analytics or AdSense.
There were blogs that explained this in detail (Facebook does something similar), but I can't find them, so here's what Google's AI overview says when I search for "How gmail cookies help google track users across the web":
Gmail cookies, such as SID and HSID, act as unique identifiers for a signed-in Google account, allowing Google to track user activity across its services and millions of third-party websites. These cookies, often lasting 2 years, link browsing behavior—like searches and site visits—to a specific user profile to personalize ads, measure campaign performance, and analyze site usage, even on non-Google sites that use tools like Google Analytics or AdSense.