Jonsior existed as a place within the galaxy. You could travel to Jonsior from the Coruscant Eastern Spaceport.
The location of Jonsior was referenced in Star Wars canon. It appeared on a manifest of destinations at the Coruscant spaceport in "Aldhani," which is the title of the fourth episode of the TV show Andor. Regarding Star Wars Legends material, the moons of Jonsior first appeared in the story Tinker, Tailor, Soldier, Praji, an article that was exclusively available via Hyperspace back in 2009.