I don't work in this industry so I don't know their secret sauce, but I would be surprised if census data is not used as a baseline for what they're selling. It doesn't make sense to not want to use it if your next best sources are relying on everyone in the household having an app that sells their location to your network constantly. I see outdated data about me on the public versions of these sites all the time, so I know they don't have omniscience.

You’d be surprised how bad/incomplete the data usually is and how much work it takes to make those datasets really useful or reliable.

That’s one reason Xoom Info was able to sell for a billion dollars and even their data has a lot of junk

Having well curated detailed census data would be a major boon for the data brokers