According to theory, there might be three types of black holes: stellar, supermassive, and miniature black holes – depending on their mass. These black holes would have formed in different ways.
Stellar black holes form when a massive star collapses. Supermassive black holes, which can have a mass equivalent to billions of suns, likely exist in the centers of most galaxies, including our own galaxy, the Milky Way. We don’t know exactly how supermassive black holes form, but it’s likely that they’re a byproduct of galaxy formation. Because of their location in the centers of galaxies, close to many tightly packed stars and gas clouds, supermassive black holes continue to grow on a steady diet of matter.