I have a question

With all of the talk about a new movie that’s just come out on Netflix called, Veronica. Which is based on a Spanish story that took place in the 90’s.

Its about a girl who looked after her siblings because her mum worked long hours. One day at school she held a séance and a nun found out and broke the oiji board. After that there was a lot of paranormal events and she died.

Now they say its based on a true story, which a lot of movies claim but they are really far removed from the real events.

The actual story, in short was about a girl who did fall ill have reportedly having a séance at school and it was interrupted ba nun and the nun did break the oiji board and afterwords the girl suffered for months with seizures and hallucinations. She was in hospital in madrid in 1991. But apparently there were no official report of her cause of death.

There is also pocile report by Chief Inspect of the National Police, named Jose Pedro Negri who claim to have witnessed disturbing events in the family home of the girl who said was "possessed" such as to have seen the door of an armoire open suddenly in an unnatural manner, and a figure of Jesus separated from a crucifix. The family themsleves reported after girl death they heard whispers in their home and experienced other unexplained phenomena including electrical anomalies and slamming doors.

My question, Just like many other movies based on true supernatural events and videos uploaded saying this person is possessed and also how the catholic church has a system to see if someone just has a mental illness or is actually possessed, Im wondering is all of it just a mental illness, and/or is it family and culture feeding the superstitious believes that actually make you sick and have psychotic episodes, how does that work or what is the mental illness?