A backstory is just that a backstory. It is notes that the novelist or screenwriter writes down to give the stories a kind of historical depth.
Like Tolkien had ages of middle Earth before you get to the story of Frodo in Lord of the Rings. A fictional backstory is a framing device of sorts. It tells you about the world its people, who they are what they are etc. These are usually only for the authors benefit and never shown.
The original 400 page Rough Draft script is the actual Blue Print for the entire star wars saga.
It stars Annikin Starkiller as the Hero. Whose Father is Kane Starkiller. Who's Love Interest is a Princess Leia but not the sister Leia. Who's father Kane Starkiller entrusts the training of Annikin to his friend Old General Luke Skywalker the protype of Obi wan Kenobi. Han Solo exists in the script as a spice runner and is a slimy alien with gills and no nose and in another draft called Jabba the Hut. The lightsaber Is called a lazersword (lucas spelling). There is an evil dictator with a wicked moustache called cos dashit. His right hand men are general Vader and Prince Valorum a Black Knight of the Sith, who in the end switches sides to join the good guys because the bad guys lack an honor code.
Lucas never actually wrote the young days of ben Kenobi or Lukes Father and just thought it might make a neat idea for a film somewhere down the line. When he mentioned this idea back in the seventies to the person Interviewing him for Rolling Stone magazine Luke's Father and Darth Vader were still 2 seperate people.