While he generally regarded it as glorified fan fiction that he could utilize to market future movies and shows, he looked at it from time to time and found works that he liked, some of which he drew inspiration from. George Lucas had an interesting relationship with the Expanded Universe. From there, the continuity expanded in multiple, sometimes contradictory directions covering nearly every imaginable angle to the universe.
While it technically began with the novelization of Star Wars, which was actually released just over half a year before the 1977 film, it didn't properly get started until the likes of Marvel's Star Wars, Splinter of the Mind's Eye, and The Star Wars Holiday Special really got the ball rolling. Star Wars Legends, alternatively branded as Star Wars Vintage on Disney+ and formerly known as the Star Wars Expanded Universe, was a collection of non-canonical stories meant to complement and build upon the worlds of the Galaxy Far, Far Away in ways that the Original Trilogy, the Prequel Trilogy, and Star Wars: The Clone Wars would not.