The parts manufacturer, in the example Hella, will produce a range of parts which goes into a range of cars. When the cars are new, the manufacturer of the parts will have agreements in place to supply certain products to certain vehicle manufacturers. For example, the original Volkswagen timing belt on my Mk6 Golf was marked up with Continental logos and the tensioner with a Gates stamp. I've bought fuel filters over the counter from Volkswagen that have come in VW branded packaging but with MANN manufacturers markings on them.
Many parts will not be sold exclusively to one manufacturer. The good old Pierburg 2E2 carburettor is a good example as these were used on many Vauxhall and Volkswagen cars.
The original manufacturer of the parts will know which cars their parts are going into and publish product catalogues accordingly. They'll also have their own unique parts numbering system and they will maintain "cross-part" compatibility charts.
A good example of this is the Gates product catalogue which is freely available online here; gatesautocat.com The Mintex Brake Book is another example.
It's in the suppliers best interest to ensure that their re-sellers are able to access this information freely because in this way, they are more likely to sell a higher volume of their product. If any parts reselling operation were setting up, they'd likely demand up-to-date product cross reference guides as part of the re-seller agreement.
So, in short, the best place to find this information is the parts manufacturers (not the vehicle manufacturers).
