https://archive.org/details/starwarstheriseofskywalkerthevisualdictionarywithexclusivecrosssections2019_202003/page/20/mode/2up?view=theater
As of episode 9's visual dictionary, they've already decided on what it actually is and the timeline of how it got built.
It's Ilum, the place where all the lightsaber crystals come from.
After ROTJ, the first order just hollowed out the entire thing and built whatever the fuck that weapon system in 30 years or less (presumably less given how much more "gap filling" disney's doing with their shows).
Abrams definitely wasn't thinking, because doing such a thing in only 30 years in secret with a significantly smaller force than an actual galactic empire is simply not possible.
