Five years after the Rebel Alliance won their battle against the Empire by destroying the second Death Star, there was a very violent civil war. Even worse then the one before the Emperor died. Many imperial forces sided with the Rebel Alliance and many Imperials formed groups against the Rebel Alliance. Many old plans and databanks for ships when the Galactic Republic was in order,
somehow surfaced. The Rebels got a hold of them first. That is where the plan for the Jedi Starfighter came from.