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> Kind of how spinning an object doesn't cause a centrifugal force, the real force is whatever forces it to stay on a circular path instead of continuing straight

Let's say the Moon is in a circular orbit around the Earth (close enough), what's the real force that's forcing it to stay on that path? If it's not gravity, what is it?

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Great question. The answer is nothing. There is no force making the moon follow a circular path. The moon "thinks" it's moving in a Newtonian, inertial "straight line." Because the spacetime around the earth and moon is curved, the moon moves in a straight line through that curved space.

Caution: The circular path we see the moon follow is not the curvature of spacetime itself. Rather it's a zero-force iso-line along that 4D spacetime. This is also called a geodesic.

The Moon moves not in circles but rather along a straight line in curved spacetime. It doesn't require any force to stay on this path — it is in free fall.

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