Hi @apotlr!
Did I understand that correctly that you uploaded the parts separately? If so then please merge the STL files which can either be done with a script or use Onshape for this which is nice as you then can use the SimScale plugin to import to the model from Onshape to your project. By the way, you can find good templates for FSAE simulation in our public projects section like this one here: Full Car Aerodynamics.
Let me know if you have any further questions and if I forgot to explain a specific point.
Jousef