I’d be curious to know where you source your data from! Your project (neat idea btw) has me thinking about tracking this data for my own personal profile over time in some sort of dashboard, to see how Google’s opinion of me changes with my behavior online

The data comes straight from Google's Ad Center (myadcenter.google.com). Google shows you the interest categories and brands they've assigned to your profile. I automated scraping that page daily for each account during the experiment.

MirrorMask actually does exactly what you're describing. It scrapes your Ad Center profile before and after each session and shows you the diff. You can watch interests appear and disappear over time. The dashboard tracks your profile changes across sessions.