me as very unexperienced game developer, I wondered how I could achieve something like re-winding the time in my game or jump back to a special moment (killcam) and replay it for the user.
The only way you can achieve this is if your game is completely deterministic. In mathematical terms, that means that if you have the function:
Then you can always get a value for
For every time
Now, this second method is obviously the best, but it's also the hardest. Because in a game, not everything can be broken down to functions:
Most likely you'll end up with a combination of the two: integration for bullet position and interpolated recordings for player positions.