Greentext

They actually are. The Moon technically doesn't orbit the earth, but a point near earth, that earth does as well. Its momentum that prevents the moon from crashing to earth, and it's gravity that prevents it from flying off into space.

https://en.wikipedia.org/wiki/Barycenter_(astronomy)

Because the earth is so much more masive than the moon, the Barycenter of the gravity point is not the centre of the earth itself.

And another interesting fact: The moon's momentum is slightly greater than that of the earth's gravity, causing the moon to very very slowly move further away from Earth (About 3.7cm a year).

but it's these momentums and gravitational forces that keep the moon orbiting (Orbital mechanics is fucking fascinating as fuck)

But what would happen if two bodies collided that are large? The force of impact would be the combined momentum of the two items as you believe. It is believed this is what actually formed our moon as early formation of the planets saw two planet sized bodies impact like you describe, the resulting force spun enough matter to form the moon (Mineral inspection of moon rock shows it contains the same isotopes as earth, which is rare if the moon formed on it's own).

pint to remember; A bodies gravitational force is based on it's mass. Larger mass items have more gravitational force. Because fo that, While gravity is 9.85m/s on earth, it would be different on every piece of matter. Even a single atom has it's own gravity force, albeit very low. Your "gravity" force on the moon is not 9.85 but a speed based on it's own mass. This is why you would be "heavier" on earth than say the moon or mars, despite your mass not changing. the planetary body mass affects gravity.

ANOTHER fun fact. Gravity and mass affects time as well. Objects closer to a center of mass operate slower than those further away. Satellites for example actually move faster in time than we do on earth. GPS for example overcame this by programming to check real earth time frequently.