Other than The Valerian, is there any other content set in the universe of The Fifth Element?
The world is so detailed, I just want to know about it. Like... What's the alien Zorg has as a pet and things like that. Are the movies really just kind of self-contained, or are they inspired/based on something? Are there maybe books set in the world?
The world of the Fifth Element is inspired from the comic Valerian, said Luc Besson.
The designs in Starwars are inspired from the comic Valerian. Georges Lucas never said it was inspired from Valerian, but you can compare : https://blog.chordian.net/2016/01/24/how-a-french-comic-series-inspired-star-wars/
Years later, Luc Besson made an adaptation of the comic Valerian. People criticized that it was plagiarism of Starwars.
If you want to get more of the Fith Element and Valérian - the movie, you could check Valérian - the comic books.