Black holes are the mass of a star compressed into a planck length(google it) creating a speck of matter that is infinitely dense. Anything to do with planck lengths or infinity should be impossible.
Actually, blackholes themselves are not infinitely dense, the singularities are therorised by current sciencentific models to be infinitely dense but no one has actually been able to test that.
As for blackholes being Planck length, those are actually possible to achieve, but not exceed, and while a few black holes might be that small most are far larger, it's not about cramming matter into a specific small space, but getting mass, and therefore gravity into a small enough space that it can self compress.
For example if you compressed the earth into a 1 cubic centimetre sphere, it would turn into a black hole but literally nothing would change because the black hole would have the same mass and gravity as the earth.
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