We don't know for sure other than what theory tells us. Inside all black holes there is a region of extreme density where the matter that fell into the black hole is crushed into a so-called singularity state. The mathematics tell us that this region has infinite space-time curvature and gravitational field strength. It is not a nice place. Between the event horizon and the singularity, the mathematics show that time and space may actually 'reverse' their roles, although what this might mean to an astronaut is unknown. A wormhole is a particular solution to Einstein's equation for gravity in which two parts of space-time may be joined together. Many science fiction authors like to use them to allow spacecraft to travel quickly from place to place in our universe. But all of these ideas are based on 'pure math' descriptions of how they might work, and as you know, nature is often much messier than any idealistic, abstract rendering. There are no perfectly straight lines in the universe, and there are not likely to be wormholes either. Wormholes do not exist naturally and would only form from certain types of black holes. So far as we understand the physics of real systems, black holes must have within them the physical object that formed them. As you enter a black hole, the object that formed it is still in front of you, filling the space-time with gravitational radiation. Wormholes do not get born this way because there is always star-stuff blocking the doorway!
