What is the leading theory on how the moon formed? Are there any other theories? Does this theory explain why other planets in our solar might have satellites too? Or is it different for each planet?

The leading theory for the formation is the "Giant Impact Hypothesis", which posits a Mars-sized planet collided with the early Earth, throwing material into orbit which became the moon.

Other possibilities have been proposed, from a planetoid captured in Earth orbit to it forming in orbit around Earth's protoplanet as a system of 2 bodies. But the Giant Impact Hypothesis is the leading theory, backed by the most evidence.