When you make a request to Google - e.g. a search - cookies are transmitted with the request and response. One or more of these cookies will be a unique identifier - we'll call it "visitor ID" - that persists for a long period. You've probably had yours set for months or even years.
This visitor ID is used by Google to tie up your sessions and activity across Google sites and other third-party websites or apps you visit that use Google's analytics or advertising services. It's critical to their operation and forms the basis for many of their products.
You said this tracking is not personally identifiable. That is (or should be) true in Europe. But you said it's not tied to a specific user. This is incorrect - it absolutely is. Without that, Google's model fails. That specific user might just be identified as a random ID, but they are the same random ID. And their profile will build, and build, and build.
Check it out yourself:
1. Visit Google in your browser, go to Dev Tools > Application > Cookies and look for 'NID'.
2. Now do the same on this page (look under 'analytics.google.com' in Cookies). Same NID.
This NID cookie is just one link to an "anonymous" profile of you. This profile will have every search you made, every page you visited, every product you bought* on any website or app** that uses Google's services. For example, it knows you read this very article.
I've oversimplified this, as there is far more than one measly cookie used to achieve this. But the concept is true.
In short, your search results are tied to you, user #192473593.
* May not understand the purchase or product itself - may just understand "bought something"
** Depends on integration, but possible
