Iron core losses: The magnetic flux in the transformer is flowing on the iron core, and the iron core has magnetoresistance to the magnetic flux, which will generate heat so that this is why open-circuit test gives iron core losses;
Copper losses: Coils are virtually made of copper wires, and these copper wires exist resistance so that they will consume a certain amount of power when the current flows through, and this is why short-circuit test gives copper losses.