The home page is a daily challenge, I've backfilled some "past challenges" so there's more than just the past couple of days to play, and there is also a free-draw mode that tells you population + some census demographic info & points-of-interest data from OpenStreetMap.
In most cases, the data comes from taking the census blocks w/ centroids contained in your circle and summing their population. Demographic info is slightly more complex as it's not sourced on blocks, so we allocate population-weighted fractions of a source territory's published census values into the drawn territory. Happy to answer any questions about the data or the technical side. The game/app only covers US areas at the moment, I'd like to add international data in the future!
I am re-submitting this as a proper "Show HN", as I left the 'Show HN' text out of the title the last time I submitted