It ends at a specific chapter, but I can't recall exactly off the top of my head. You might want to check online manga forums or dedicated One Punch Man fan sites for the precise chapter.
You can find 'One Punch Man' fanfictions on websites like FanFiction.net. It has a large collection of fan - created stories related to 'One Punch Man'. There are different categories and ratings, so you can easily filter and find the ones that suit your taste.
One Punch Superman was finished. Although the plot of the first season was very exciting, the animation production company did not seem to have any plans to continue the second season. However, fans of One-Punch Superman could still continue to enjoy the story through comics, novels, and other media.