"Aftermarket" is a term used to describe accessories that are not made by the factory. So for example, for your Mazda Millenia, if you buy accessories for your car that are not actually made by Mazda, then those are termed "aftermarket". Many people use the term "Millenia aftermarket accessories" specifically when referring to custom accessories for the Millenia, like performance accessories or custom appearance accessories. But even if you just replace your Millenia brake pads with OEM-grade factory-style brake pads that are not made by Mazda, those too are aftermarket Millenia 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".