All they really had to do was cross-reference several databases with ordinary computers.
Voter Registration Database
County Property Appraiser’s Residential Addresses and Owners Database
County Coroner’s Death Certificate Database
County Building Permits Database of permits issued but not yet built and occupied addresses............
I would hazard that by no means all the thousands of jusrisdictions involved have the same data structures and formatting.
It's a big job. I'm glad somebody is attacking it.
In practice, commercial data vendors make a good living from acquiring and cleaning up raw data from many sources around the country. And they are willing to provide access to that data -- for a price. Even then, for the project at hand, some unique, non-commercial political data sets still have to be found, acquired, cleaned up, and processed for insights.
In the end, it comes down to not just computers and software, but also time, talent, and cash.