how do they do it?

Yep, more than likely, a couple of different databases.

There's probably a "template" page with fields for text, specifics on the car, pictures, etc. that are then populated by calls to a database depending on which link you clicked on.
 
Yeah, any page that ends with "asp" or "php" is dynamic, meaning its data is generated with the latest data every time it's requested (well, there's surely some amount of caching involved to reduce server load, but you get the idea). It's the same way (most) forums work; if you change your sig or avatar, that change will be reflected in all your previous posts, because the forum generates its pages dynamically.
 
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