How did the asteroid belt form?
Short answer: The asteroid belt probably formed from several "failed planets" and a bunch of smaller stuff.
Long answer: Astronomers have spent many years trying to figure out exactly how the asteroid belt was made. There have been many theories, including an early popular theory that the asteroid belt consists of the remains of a planet that disintegrated millions of years ago. However, we have since learned that the asteroid belt doesn't contain enough material to account for a planet. Also, asteroids in the belt are made of different chemical elements which would not happen if they all came from the same planet.
In 2018, researchers from the University of Florida found the asteroid belt was more likely created from the remnants of several planets instead of a single one1.
In the early days of the Solar System, small planets (proto-planets or planetesimals) began forming from clumps of solid material. In the region between Mars and Jupiter, it appears that one or more planets could have formed, but the gravity from Jupiter kept pulling things apart. Instead of forming a single planet, the material in this region was only ever able to form into asteroid-sized objects.
Footnotes
1. "Study reveals secret origins of asteroids and meteorites". news.ufl.edu. 2018-07-02