If you take a creative writing class you’ll find it is very common to give characters agency beyond what the author wants (e.g. I was going to have her do this, but she disagreed and did that…) and part of that is building a larger world, creating backstory for the characters. A great example is in the Quentin Tarantino film “Once Upon a Time in Hollywood” the main character stars in a Hollywood wester TV series. Even though the movie only shows a tiny bit of the series while they’re filming a couple scenes, apparently Tarantino wrote several full episodes of the hypothetical series. This kind of backstory writing seems to be pretty common. (My MFA is in creative writing, so I spent a lot of time with other writers.)