That is correct; every car that goes into Forza takes considerable time & effort to build, test, tune, license, etc. Some more than others. Forza 6 shipped with over 450 cars, many of which were updated from Forza 5 or Horizon 2 (a lot of work by a lot of people), many of which were updated from Forza 360 games (considerably more effort in terms of time & resource), and many of which are brand new to the series (a massive effort taking many months per car).
None of these cars get magically ported from one game to the next with the press of a button or running of a script. It takes artists, engineers, audio designers, gameplay designers, testers, and licensing specialists a lot of time and effort that most of you don’t think about, and really shouldn’t have to, when you get behind the virtual wheel and take them for a spin. Track or world expansions are all that effort multiplied by a very big number.
All that effort mentioned above sometimes comes with additional cost to the player (buying the full game or buying DLC car packs or expansions), and sometimes no cost (free cars or free expansions).