I think most of us would be very surprised how many different manufacturers will produce aftermarket parts.
For the last 20+ years of my career, I worked for a company that manufactured automotive safety components, such as seat belts and airbags. We were a Tier 1 / Tier 2 supplier to most all major auto manufacturers in North America.
We only sold our product directly to the auto manufacturers, or in less common situations, to another manufacturer that installed our parts into a subassembly that was then sold to an auto manufacturer. But even in these instances, the auto manufacturer paid us for the parts that we shipped to the other supplier.
To be clear, we never sold any of our parts to anyone other than the auto manufacturer. We never sold to aftermarket auto parts sources. To do so would have been a major violation of our contracts with the auto companies, and could have resulted in loss of hundreds of millions in contracts.
And yet, there were, and are, plenty of sources out there for aftermarket airbags and seatbelts, some of them even being counterfeit, being marked with our company branding. Knowing how much our company invested in tooling up for production, it always shocked me that other companies could tool up, just to build aftermarket parts, and be profitable. But they do.