We have been to the moon and collected samples. Regardless of the why, we know it has a similar composition to the Earth. At least at the surface; seismic data

We have been to the moon and collected samples. Regardless of the why, we know it has a similar composition to the Earth. At least at the surface; seismic data suggests, for instance, that its iron core is comparativly smaller than Earth's. All of this is inputs that led to the impact model of moon formation.