From what I've heard, dinosaur bones tended to be taken for the bones of giants, at least in the Middle Ages. Dragons were much older than any awareness of dinosaurs; most medieval myths about dragons trace back to the draco standard carried by late-Roman cavalry. (See https://en.wikipedia.org/wiki/Draco_%28military_standard%29 .)
This, by the way, means that there really were dragons in sub-Roman parts of the Dark Ages world: Wales and Cornwall, for example, where draco-bearing cavalry (including King Arthur's, if there was a King Arthur) were fielded probably until the 600s or 700s.
Dragon mythology as an explanation for dinosaur bones seems pretty straight-forward. I'd even say that "dragon" was just the early word for "dinosaur", and the mythology is based on an earlier extrapolation of characteristics from fossil remains than what we use today. A somewhat less strict extrapolation, but even today a lot of what we 'know' about dinosaurs is pure guesswork.
But dinosaur bones driving myths of biblical humanoid giants? That's a much bigger stretch. Most dinosaur bones are pretty non-humanoid, and people who are generally a lot more familiar with human and animal skeletons than we are today wouldn't mistake one for the other. Also, while dragon mythology is mostly reasonable based on fossil evidence (except for the flying and fire-breathing... and gold hoarding) giant mythology is completely different. The biblical texts and non-biblical books like Enoch give them names and actions that are much more historical sounding than mythological. You don't get details like that from big leg bone fossils.
So, I don't buy dinosaur bones as the source of giant mythology.
I have no idea where dragon myths or giant myths originate; giants are easy to guess, but dragons are a strangely common belief for creatures with so little basis in reality. (Plasma cosmology has a very interesting explanation for dragons, and for a lot of other strange things in human history, but the physics of that theory are pretty bad.) I'm just mentioning what it is that dinosaur bones were identified with in the Middle Ages -- centuries before anyone knew that there had ever been such things as dinosaurs.
(To an extent, dragons fall into the same category as unicorns, griffins, and phoenixes: not mythology so much as really bad zoology. All four of these species were believed to exist in the real world as contemporaries of medieval humanity, sometimes surprisingly nearby -- like griffins in the Caucasus.)
This, by the way, means that there really were dragons in sub-Roman parts of the Dark Ages world: Wales and Cornwall, for example, where draco-bearing cavalry (including King Arthur's, if there was a King Arthur) were fielded probably until the 600s or 700s.