If you read that book I posted the title to....Dr. Angell MD explains it completely.

I read it a while back, so I don't want to paraphrase it. But I seem to recall that the "rights" to the drug are leased from the discoverer. Now when NIH is involved, it can become political, and I don't know the nuances of that.

Suffice it to say, Big Pharma is mostly a marketing tool, today. Back when I started in this area, they actually discovered chemical products and patented them. But that is not true today.
For example... the metabolic by products of drugs are done today as new entities by a separate company (companies). Examples are Clarinex (from Claritin) and Xopenex (from albuterol). I am sure there are others now but Sepracor (which is going under or has by now did those two.). The Big Pharma labs are becoming extinct as I type this.

I do think they do some dosage form manipulation. Oral forms, etc, plus the marketing still. But I do know that chemists in some large pharm companies buy the rights to drugs made in Japan and Northern Europe commonly.