"Aftermarket" is a term used to describe accessories that are not made by the factory. So for example, for your Alfa Romeo Giulia, if you buy accessories for your car that are not actually made by Alfa Romeo, then those are termed "aftermarket". Many people use the term "Giulia aftermarket accessories" specifically when referring to custom accessories for the Giulia, like performance accessories or custom appearance accessories. But even if you just replace your Giulia brake pads with OEM-grade factory-style brake pads that are not made by Alfa Romeo, those too are aftermarket Giulia accessories. Over the course of a vehicle's life, it almost certainly will have aftermarket accessories installed in it at some point, even if the vehicle is never actually "modded out".