There is a difference between an OEM and a counterfeiter.
Every spring in the world (Practically) is made by Eibach. If you buy a car spring (In particular) it’s just OEM from Eibach with a different name on it.
Look at Tenneco in NY (Rye). Those guys make every damned muffler there is.
I have read that there are only a handful of optical glass makers in the world.
I think Schott in Germany is the largest. Nikon makes their own and there are probably a couple more in Japan. Rochester New York used to have at least one, maybe more but I suspect there are not any there now.