With an impact like this, it’s probably more correct to say that the impact of Thea and Earth formed two bodies, that we now call Earth and Luna (the moon). Earth has a lot fo Thea and the thing formerly known as Thea has a lot of Earth. They basically become two liquids impacting and swirling around. The cores are largely the same, but the sarfaces are a bit of a mix.
With an impact like this, it’s probably more correct to say that the impact of Thea and Earth formed two bodies, that we now call Earth and Luna (the moon). Earth has a lot fo Thea and the thing formerly known as Thea has a lot of Earth. They basically become two liquids impacting and swirling around. The cores are largely the same, but the sarfaces are a bit of a mix.