In reality, there are only a few manufacturers of spark plugs that supply the vast majority of the entire market. Branding at the time of manufacture is very common in many industries and spark plugs are no different. It’s kind of funny when you hear someone as a staunch advocate of a particular brand when those plugs are actually made at the same plant as the ones they are disparaging.
There was a cartoon once that showed all the various mail slots at the post office from the customer perspective and then from the other side, they all fed into one bin.
You could almost turn that around to depict spark plug manufacturing…