The way most most Supermassive Black holes form is by "eating" other black holes, stars and other things. Everything that the black hole consumes adds more mass to it and the black hole grows. The more it eats, the bigger the black hole becomes. Most black holes start out as small ones. Then they suck things in it and they become bigger.
The way they form in the first place is:
A giant star runs out of Hydrogen and other forms of fuel. It collapses and what's left over is a black disk that has an immeasurable amount of mass. In fact, according to Einstein, a black hole might have an infinite amount of mass. However, I find that hard to believe, because it sounds impossible for anything to literally have infinite mass.
The Sun is a Medium star. It doesn’t have enough mass to form a black hole. Scientists measure other stars, black holes and other really large objects in space with. "Solar. Masses". One solar mass is the mass of the Sun.
In order to form a black hole, a star has to be at least 5 solar masses.
There are a few black holes that Scientists have found which they think are Millions or Billions of solar masses. These are called: Supermassive Black holes. There is one in the center of every spiral galaxy, including the Milky way.
The gravity from these Supermassive black holes helps hold the galaxies together.
Scientists are not sure exactly how Supermassive black holes form. However, one reason might be that it could of consumed Thousands of other black holes and planets and kept getting bigger.
The gravity from a black hole is so strong that almost nothing can escape from it. Even if something traveled at the speed of light, it would not be able to escape from the gravitational pull of a black hole. That is why they are hard for Scientists to find.
However, Stephen Hawking discovered a type of radiation that does manage to escape from black holes. Since he discovered it, this radiation was named: Hawking radiation. Astronomers need special, X-ray telescopes to see it.
This technology is so new, that until recently, nobody had ever actually seen a black hole. They knew they existed, because they could see stars, planets and other things getting sucked into them. Hopefully, with the newer, X-ray telescopes, Scientists will actually be able to get a picture of a black hole for the first time ever! Astronomers have been talking about trying to do that this year (2018).
https://www.sbs.com.au/news/dozen-black-holes-discovered-at-the-milky-way-centre
