Hi there! I'm not sure if this will be particularly useful, but I'll give it my best shot!

Think about what drives your story.

If it's science fiction and follows a broken world split into class systems fighting for dwindling resources after a disaster (but the story starts after the disaster has happened), your plot background could be the events leading up to the disaster, or the disaster itself, or its immediate aftereffects, or how one of your characters (much younger) first interacted with it.

There's a literary science fiction novel called Station Eleven, for example. A horrible illness wipes out much of the population (eep, sorry; bad timing—but it's an effective example!) and it follows a traveling group of performers in the post-apocalyptic world. It has many POVs. Much of it takes place in the present, after the world ended, but scattered throughout are snippets of what life was like before—a young girl who had a small part in a theater production just before the illness started spreading. A woman in a tense marriage putting her creative energy into a comic that somehow survived the apocalypse in the hands of one of the other characters. The last moments of some of the characters who didn't survive but had a powerful effect on the story that follows. Things like that. Any of those bits in bold would work as a "plot background story," especially in a character-focused literary work. Now, for a more gritty genre-fiction thriller, a plot background story for the same premise could be a day in the life of patient zero or something like that.

If it's a contemporary drama that has a lot to do with the protagonist's alienation from his family/friends/extended family/a significant other but starts years or decades after whatever separated them, your plot background could be the argument/event that tore them apart... but it could also be anything else that feeds meaningfully into any of the things that drive your story.

For stories that have a single, high-concept inciting incident, it might be easier to write a plot-background than for stories that are more character-focused, but it can definitely be done! And I don't think there's always one right answer. If several major conflicts or sources of tension feed into your story, you might choose just one of them—whichever one speaks to you the most—and use that one for the plot background story.

I hope this makes some sort of sense!