In the end, Theia’s debris gathered together around Earth to form what was the early Moon. After the debris from the collision flew into space, scientists think that it originally formed 2 moons. The moons later merged to form the single Moon we know today.
While the Theia model, known as the Giant Impact Hypothesis, is the leading contender, it’s not the only one. Maybe the Moon was just captured somehow by Earth’s gravitational pull. Maybe the Earth and Moon formed around the same time as a pair. Maybe something else.