Through building our MVP we've gone through the act of finding out what companies like Meta and Google store about us. We found some of the obvious stuff, but also some of the more interesting data, like Google's predictions about the way you travel.
Some of these finding might be interested to the r/datahoarder type folks who like being in control of their own data, or just to interested hackers who like a high-level glimpse into what these companies are collecting.
If there's any interest in a deeper (and more technical) dive into what these companies gather (my google takeout was several GBs big), I'd be happy to write one up.