Through many years of advertising, a perception has been created that the parts supplied by car dealerships are "genuine", thereby implying that the parts sold by the aftermarket are inferior "copies".
A supplier might win the right to supply a specific part to a car manufacturer for a specific model. That supplier might have several original equipment (OE) contracts in place with various car manufacturers. This is the nature of the supply chain where car manufacturers source parts and components from multiple suppliers depending on price and market logistics.
A little-known fact among the general public is that most OE suppliers also supply the aftermarket.
Some suppliers have contracts with more than 20 different car manufacturers and have been OEM for many decades.
Where the wool has been pulled over consumers' eyes is that aftermarket distributors often sell precisely the same parts used on the production line. Still, instead of supplying them in boxes with the car manufacturer's logo, they are sold under the brand of the manufacturer of the part – a "private label" if you will.
Of course, some parts manufacturers are not OE suppliers.
Some have chosen that path because it is more lucrative to supply the aftermarket, while others do not have the quality to meet the car manufacturer's stringent demands. It is this latter group that sometimes leads to the reputation of aftermarket parts being inferior to the parts that car dealerships supply.
Our advice to car owners and consumers is to purchase parts from companies with a reputation for quality and service.