No man, I think you are not importing game ready content. If you are importing a high poly car you downloaded from Turbosquid, in example, if it is not a game-ready model, if it is too much high poly you wont be able to import in any game engine, as for games the model needs to be optimized.
You can keep the high poly details by baking normal maps, which will get all the surface details and render it to a texture (normal map), then you use this normal map on the material of the object, so you will have all the high poly detail on a low poly surface.
This is how 3d assets for games work. People begin by modeling a high poly and high detail model of a object (generally in Zbrush), then they extract normal maps from the high poly mesh, and then there is a lot of tools on 3d softwares (3dsmax in example), to reduce the number of polys without “breaking the mesh”.
I use ProOptimizer (https://knowledge.autodesk.com/suppo…39FB3-htm.html), which is a modifier (plugin) for 3dsmax and it does an excelent job for reducing polycount on models.