What process caused the iron-nickel core to form in the early Earth?

The sinking of high-density iron, originating from ancient supernovae debris, toward the center upon accretion.

The iron present abundantly in terrestrial planets like Earth originated from the cosmic leftovers of previous generations of massive stars that lived and died, forging that iron via fusion before the Sun ignited. When the early Earth was forming, this accumulated cloud of gas and dust contained this stellar debris. Because iron possesses high density relative to the surrounding silicate materials, the massive amounts of iron present migrated gravitationally toward the planet's center during the accretion and differentiation process. This gravitational sorting mechanism resulted in the formation of Earth's distinctive, dense iron-nickel core, linking our planet's fundamental geology directly to the life cycle and death of ancient stars.