Black holes are a singularity in space-time at which our best theory for gravity and quantum mechanics completely breakdown. Everything becomes infinitely dense and time essentially stops. As Einstein discusses, nothing not even light can escape a black hole at the event horizon. At this point, it is impossible to tell whether things are ‘happening’ as there isn’t any point of reference.
If you fell into a black hole, you would be spaghetti-fied. Your legs and body would be stretched infinitely. Your feet would move at relatively different speeds to your head so different parts of your body would be travelling through space-time at vastly different rates – your body would take travel in different parts!
Based on our theoretical understanding, black hole are not portals to another world. But, we simply do not know as it is impossible to send probes into a blackhole and get measurements back out. Also, you can’t see black holes, so they are very hard to find!
However, wormholes (Einstein-Rosen bridges) are theoretically possible. These are ‘short-cuts’ through the curvature of space-time (caused by very massive objects). If you can take a route which is shorter than the route light takes (the fastest anything can go), starting from the same point and beating it, you are time travelling!