Between Google Search, AdSense, Google Analytics, Google Tag Manager, and various other APIs, Google has all the pieces in place to get a pretty good overview of what users are doing online. (And that's not even considering that the user's likely to be logged into a Google browser that's uploading their browsing history wholesale.)

Legally speaking, Google's not supposed to be correlating most of this data. But, as we know well, that doesn't mean they aren't doing it.