Sounds like AllAdvantage (https://en.wikipedia.org/wiki/AllAdvantage). An inspiration in part to Brave, but of course it did not blind itself to user data as we do.

The entire Google ad supply does not need to be downloaded, just a relatively brief catalog (which compresses well) of live edge URLs and metadata (keywords, essentially), updated as new deals for a given region with large enough user base come online, and old deals expire.