There are things that spark a story. An image. A sound. A memory. Sometimes there's guidelines – some editor is looking for commonly themed stories, and it starts by riffing on that theme, until I find that piece that's the beginning of the story. That's kinda like jamming – get some notes out, some chord changes, until you find the groove. And when you do, then it's not you making the music. The music is always already there, and it has chosen you to be the conduit to bring itself to life.
Sometimes, that spark is as far as it gets.
I have lots of files that are one sentence, or a description of an image or feeling. I have lots that are a paragraph. Or a page. Lots where I was never able to find the rest of the story.
Other times, it's the beginning of discovery.
The story is there, if you can find it. If you can bring it to light. If you can uncover it. The story is there, and the characters are as well. And the story is true. As a writer, your only job is to be a conduit for the story. You are the media that the story uses to organize itself, to put itself into text, onto paper or pixels.
And you can try to shape it, try to bend the story and characters to your will. Make them do what you want them to.
But then you're no longer writing something true.