Sagittarius A* is such a supermassive black hole that it registers as .0003% of the Milky Way’s total mass. For perspective’s sake, the total mass of the galaxy is more than 1.5 trillion times the mass of the Earth’s sun, making the black hole in question 51.8 million kilometers in diameter.
How the supermassive black hole was originally formed is likely similar to others. When a giant star burns out and collapses on itself, the result is a mass so dense that not even light can escape it.
These mysterious bodies will suck in anything that gets too close, including other black holes that might orbit a little to close.