In some stories, the area under the stairs can be seen as a sort of in - between space. For the 'boy under the stairs story', it might mean that the boy is in a liminal state. He's not fully part of the main living area upstairs or downstairs, but in this odd, tucked - away place. It could symbolize his isolation from the normal life around him.
The location under the bridge could be a place of isolation. It might signify that the girl is in a marginalized situation, away from the normal hustle and bustle of society.
Under story fanfiction is basically when fans write stories that are related to an existing work but focus on the less - prominent elements. It could be about a sub - plot that didn't get much attention in the original. For instance, if there's a detective story, the under story fanfiction might be about the personal lives of the forensic team that assists the detective, which was just a minor part in the original but has a whole new world of possibilities in fan - made stories.
I'm not sure. It could be a blend of real and made-up elements. Without definite information from the creators, it's difficult to determine if it's a true story.
I don't really know. To determine if it's a true story, you might need to research the source material, reviews, or interviews related to it. There could be elements of truth mixed with creative license.