How did gravity and density affect the order in which these spheres ended up on earth

The atmosphere and Earth's interior are layered by density. The differentiation between these layers is due to processes that took place during the early stages of Earth's formation (ca. 4.5 billion years ago). At this time, melting would have caused denser substances to sink toward the center while less-dense materials would have migrated to the crust.

Gravity pulls more strongly on denser materials so denser materials are at the center of things.

Explanation:

Gravity pulls more strongly on denser materials so denser materials are at the center of things. The lowest layer of the atmosphere is denser than the upper layer. Earth's liquid metal core has convection, which generates the magnetic field.

i do not get it   at all