Where did Earth's Iron core come from and how did the mantle become molten?

So recently I was told by a creationist as a rebuttal to the various and often fatal heat related arguments against YEC that the Earth was not molten, and that it was "void and empty."Now, I'm going to assume that was true. How, outside of magic, did the iron core of the planet get there and how did the mantle become molten if during the creation of the planet, Earth wasn't molten at all, but "void and empty?"What geological principle or phenomena could explain this?