Stories are fractal by nature. Even when there’s just one version in print, you have it multiplied by every reader’s experience of it in light of who they are, what they like, what they want. And then many people will put themselves in the place of the protagonist, or another character, and spend a lot of time thinking about what they’d do in that character’s place. Or adjusting happenings so they like the results better.
That’s not fic yet, but it is a story.
But the best stories grow. This can happen in the language of capitalism—a remake of a classic movie, a series of books focusing on what happened afterwards or before—or it can happen in the language of humanity. Children playing with sticks as lightsabers, Jedi Princess Leia saving Alderaan by dueling Vader; a father reading his kids The Hobbit as a bedtime story as an interactive, “what would you like to happen next?” way so that the dwarves win the wargs over with doggie biscuits that they had in their pockets and ride to Erebor on giant wolves, people writing and sharing their ideas for deleted outtake scenes from Star Trek and slow-build fierce and tender romance with startling bursts of hot sex between Hawkeye and Agent Coulson.
A story at its most successful is a fully developed fractal, retold a million times and a million ways, with stories based on stories based on stories. Fanfic of fanfic of fanfic. Stories based on headcanons, stories based on prompts, stories that put the Guardians of the Galaxy in a coffee-shop AU and stories where the Transformers are planet-wandering nomads and stories where characters from one story are placed into a world from another. Stories that could be canon, stories that are the farthest thing from canon, stories that are plausible, stories that would never happen, stories that give depth to a character or explore the consequences of one different plot event or rewrite the whole thing from scratch.
This is what stories are supposed to be.
This is what stories are.