If their mass is always contained in a singularity and their density is infinite, how are some black holes supermassive and others are not?
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The size of a black hole depends on its mass.
A guy very fittingly named “Schwarzschild” (black shield) worked that out some time ago.
If you increase the mass of a black hole, by for example dumping some stuff into it, you increase its size by a corresponding factor.
The exact formula is that the radius of the event horizon equals the mass of an object times 2 times the gravitational constant divided by the square of the speed of light.
Since the speed of light, the gravitational constant and 2 are all constant, the radius grows proportional with the mass of an object.
You can do that in reverse to, to figure out how much you would need to compress an object to turn it into a black hole. Our planet would need to be compressed to a radius of 8.87 mm to turn into a very small and not very long lived black home.
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