[Handy Hanafiah]

no need to create new folder, the cars/tracks mod should be put at assetto corsa main directory along side with stock assetto corsa cars/tracks.
folder for cars: Steam\steamapps\common\assettocorsa\content\cars
folder for tracks: Steam\steamapps\common\assettocorsa\content\tracks

example of mod car sharing same folder with stock asseto corsa cars (lada and ks_)


some track may require you to have shader patch
i highly recomended to use content manager as launcher and shader patch from there.