Mirror modifier?

hello, I am working on a car so far / to make my first own model, I modelled just the half of the car because there is the mirror modifier in blender, it works with the windshield/hood good but other things like lights are directly next to themself so they dont spwan on the other side of the car if I add the mirror modifier, btw I wantet to load the model in game but just the jbeam; not the mesh is showing up... is the workflow btw wrong or is it right how Im doing it, I know i need sth like a main body but I just startet with the easier things like hood etc