Built by one person, in the open
A comparison is only worth as much as the numbers behind it, so every spec comes from the manufacturer itself: its spec sheets, press kits and model pages, not a reseller's database. Fuel economy and crash ratings come from the EPA, NHTSA and IIHS. Each figure is stored with the document it came from and the model year it was printed for. When two sources disagree, both stay on the record until something settles it. When I've got something wrong, the correction goes on the page rather than quietly into the data.
That is slower than pulling a feed, and it is the reason the guides can tell you things other sites can't. A sunroof costs between 0.3 and 2.3 inches of headroom depending on the car. On most of them the cheapest trim is the roomiest. Neither of those shows up unless somebody reads the sheets.
The 3D builder gets the same treatment. Every car is modelled by hand, with the body panels, glass, lights, wheels, brakes and interior all cut apart so each piece can be swapped on its own.