I'm in marketing, with extensive email marketing experience. You'd be surprised at how much we can track based on a single open of your email. Even better is when you click a link with all the tracking info included in the URL string.

There's also platforms like HotJar that can look at someone's browsing habits in real-time if they happen to be on a particular website that has their script installed. Admittedly, I haven't used HotJar within the last two years or so...maybe they changed that feature by now because of privacy concerns? But yeah, back in the day I could see exactly where visitors were on our website and every action they took. You can also see larger amounts of visitor data in a heatmap format in order to see things like scroll depth, if users are clicking what you want them to click etc.

You'd be surprised at what can be tracked...