Dragons are just Bigger Swallows

Kojiro had been through a lot of weird things. At first, it was coming to terms with the fact that he wasn't real. Then, there was being summoned as a heroic spirit by Medea and then by Chaldea. However, this situation was by far the strangest. Why was he suddenly inhabiting the body of some orange haired kid? And also... why weren't there any swallows in this world?