Do we even know what a black hole is?
Well, yes and no. We know that it is a region of such great energy density that it has a gravitational field so great that nothing that passes a certain distance from the center of it (known as the event horizon) can escape. So that’s the yes part. In the equivalent terms expressed in the language of relativity, it is a region of highly curved spacetime such that the ordinarily space-like coordinate that describes the distance from the black hole center becomes a time-like coordinate instead. This means that not only can nothing escape once it crosses the event horizon, it cannot move freely in any spatial direction, but instead is constrained to fall ever closer to the center. Essentially anything crossing the events horizon falls forever.
That’s the “yes” part. We can make these conclusions based on the Einstein field equations of General Relativity, so we are fairly confident in them. Not only do observations such as these conform to what is predicted by GR, ALL observations of systems in which gravity plays a significant role conform to the predictions of General Relativity. There are no exceptions (as of now). The consequences of GR seem weird, but that is because our normal experience is in an environment where an approximation to them, namely Newtonian gravity, holds to a high degree of accuracy. While the two agree in most everyday situations, there are measurable (but not immediately apparent) effects where they differ, and in all such cases relativity gives the right answer.
Two good examples: very accurate clocks will show different times on earth and in a weaker gravitational field in orbit. This is accounted for by relativity and absent n Newtonian gravity. We know it occurs because our GPS systems require very accurate time measurements to give accurate locations, and the clocks in these systems must be adjusted to account for the gravitational field in the GPS satellites. Another example, if you stand on the ground and shine a light toward the top of a tall building, the light will have a different color when it gets there than it did when it started out. The light will be redder at the top of the building - that is it will have a lower frequency. This effect is too small (and our buildings are too short) to observe.with visible light, but it has been measured with gamma radiation (which has a higher frequency and thus a greater effect).
Now the “no” part. Since nothing can escape from the event horizon, we cannot observe anything inside it. We cannot tell what black holes are made of. We cannot tell what is happening to anything inside the horizon. The observations about curved spacetime and objects being forced to fall forever are from the reference frame of an outside observer. The equations of relativity, as confident as we are about them, simply cannot be applied to the region inside the event horizon in a sensible way. It is fair to say that we don’t know what is inside the black hole. As it turns out, all we can tell about a black hole is its mass, electrical charge and angular momentum. Anything else is impossible to know.