Heat from a thousand impacts
Growing this fast is violent. Every body you swallowed struck with the energy of its fall, and that energy stayed as heat. Add the warmth of radioactive elements decaying deep inside, and you crossed a threshold: you began to melt. You are a protoplanet now, glowing softly in the dark.
Iron finds the center
Molten, you sorted yourself out. Heavy iron and nickel sank toward your middle while lighter rock floated up. This is differentiation, and it gave you a dense metal core, a rocky mantle, and, in time, a magnetic field. You stopped being a pile of rubble and became a world with an inside.