A little about black holes
Black holes (BH) do not have tangible boundaries or forms, they have no real surface. However, they have a clear boundary that can be calculated, and if we were to fall into it, we could determine the moment of crossing this boundary. It is called the event horizon.
If we take any object, it does not collapse into a BH because of electromagnetic forces. It's also because of them that we don't fall to the ground when sitting on a chair, and don't fall through to the center of the Earth when we find ourselves on its surface.
Stars are primarily prevented from collapsing into BH by pressure from thermonuclear reactions (TR) and expanding gas, which oppose gravity. When TR are finished, the core begins to compress. There are several options for what forces can kick in and stop the compression. But if the star is massive enough, these forces won't be sufficient and it will compress into a BH. The Sun, for example, is not massive enough and will compress into a white dwarf. These are stars in which TR have stopped.
If an object falls into a BH, tidal forces will act upon it. The smaller the BH, the greater the tidal forces. Their effect is that the part of the object located closer to the BH will be attracted to it more strongly than the part located farther away. This is also called spaghettification. For BH of stellar mass, such an effect would already be noticeable. Falling into a BH doesn't happen in a straight line — the object rotates around it, gradually approaching, and eventually reaches the singularity.
BH have a hypothetical radiation process called "Hawking radiation." The smaller the BH, the more it radiates. The main radiation lies in the invisible spectrum. If we take a massive BH, we can say that its radiation is very, VERY dark red, and the human eye will not register any photons. What then do we see in a photograph of a BH? This is matter that moves around it. The event horizon itself is located deeper and is visible as that very "hole."
What can a BH be made from? Simply from mass, any kind, or radiation. You can throw anything into it and it will become part of the BH. At the same time, almost all parameters of the original matter disappear in it. In general, a BH can be described by two parameters: mass and rotation. Mass is significantly more important.
BH do not make a large contribution to the formation of the universe's structure; probably it could do without them. For now they are important to us as certain laboratories, for example, for building quantum gravity theory.
We still don't know how BH are structured. Theoretical physicists give various answers to this question.