Cool. Do you have a quick reference for the 'landed' part? As opposed to being part of the planet from the beginning. Thanks!

https://www.oakton.edu/user/4/billtong/eas100/cosmicorigins....

Is it because all heavy elements present when Earth formed sank to the center?

That would make sense, but I've assumed that since we can mine gold, that's not what happened. If all the gold fell from the sky, that would explain things.