The physics of spinning black holes explained

Very Interesting article. It got me thinking and so I throw this out mostly as a question:

The idea that angular momentum must be conserved and that it “can’t be destroyed” is based I think on Newtonian physics for the most part. But does that miss the concept of mass converting to energy, which may very well be a part of that transformation into a black hole. In that cases would not the momentum change / reduce in that simple framework. From my vantage, I don’t believe we know anything about what is going on with mass inside a black hole. We can measure the effects of the space and energy around it, and using mathematics, estimate what the mass (effect) is, but we have no way of measuring its actual mass precisely to my knowledge. We can with planets and stars, as they are somewhat “stable”, relatively “small” masses when compared to the distances used in the calculations. They may even be able to be reduced to point mass for the purpose of estimation. But I would think a black hole is a whole different thing when actual physical mass is an unknown.