It was set up carefully in the '90s when the Thrawn Trilogy was released. Much of the technology and lore was drawn from the old WEG RPG sourcebooks which in turn drew from the novelizations of the films and the earliest book tie-ins. GL even had some influence over parts of it. He gave notes to Zahn about the Clone Wars, he vetoed the Clone Vader storyline of Dark Empire and suggested a Clone Palpatine instead, and lastly he shot down killing off Luke Skywalker in the NJO series. A team also was put into place to ensure future books built off of one another and didn't conflict with one another. It wasn't until the Prequels came about that alot of retconning had to take place but they didn't start from scratch.
In 25 years there will be so many books, comics, video games, and movies this new canon will become as convoluted as the last and someone at Disney/LFL will find it easiest just to hit the reset button again now that they've done it once.