Though much of the plot of Star Wars: The Rise of Skywalker was kept secret right up until the opening crawl, one thing the film's marketing put out in the open almost from the beginning was that Emperor Palpatine was back, and he had a gigantic plan for some kind of final victory. When Star Wars fans finally saw the film, we got to see exactly what that plan was and how the once-dead Emperor wanted it all to unfold, but there were still lingering questions, particularly about that massive fleet of Star Destroyers he'd managed to marshal. Thankfully, the film's Visual Dictionary is here to provide a few answers.
**Spoiler Warning: There are spoilers for Star Wars: The Rise of Skywalker below.**
Even before we saw the film, we knew that Palpatine's return somehow involved a massive fleet floating in a foggy sky. The Rise of Skywalker tells us that this planet is Exegol, a hidden Sith bastion, and that the fleet is Palpatine's Final Order, a massive secret force anchored by countless Star Destroyers, each equipped with a planet-killing superlaser. The film uses the Final Order fleet as the centerpiece for the climactic battle, but much like the circumstances surrounding Palpatine's physical return, it leaves the how of the fleet's creation and operation to the imagination.
The mysteries surrounding the Final Order are there thanks, in part, to the simple limitation of the film's runtime. There's simply not space to stop and explain where the ships came from, who's crewing them, and why. There is, however, plenty of space to talk about that in The Rise of Skywalker Visual Dictionary, written by the Lucasfilm Story Group's Pablo Hidalgo.
While the movie doesn't ever tell us exactly how Palpatine was able to return from the dead, it does show us that he did it with the help of a group of mysterious, hooded followers who were devoted to his vision for the galaxy. According to the Visual Dictionary, these people make up a cult called the Sith Eternal, and it's their fanatical devotion and influence that's allowed Palpatine's return to come with such force. The Sith Eternal were able to revive their revered Darth Sidious through a merging of "technology and the occult," and while we don't know exactly when Palpatine came back, the Visual Dictionary does tell us that the cult has spent "decades" on Exegol crafting their beloved Emperor's new fleet.
The film tells us the why behind the fleet, but not the how. According to the Visual Dictionary, the Star Destroyers on Exegol are not leftovers from the days of the Empire, but a new class — the Xyston-class Star Destroyer — manufactured by shipwrights, engineers and slaves under the yoke of the Sith Eternal on Exegol. The cult had three decades after Palpatine's fall in Return of the Jedi to build the ships, and it's clear that they wasted no time, but where did they get the parts? There's an answer for that too: The Sith Eternal is made up of some very influential people in the galaxy, including people who served on the boards of Sienar-Jaemus and Kuat-Entralla shipyards. These titans of galactic industry were able to smuggle the parts needed for the fleet out of those facilities and onto Exegol, providing the raw material necessary to build the fleet's might.
Even after the ships are in place, though, you can't move them without crews, so how does that work? Well, you may have noticed in The Rise of Skywalker that, with the exception of commanders like Allegiant General Pryde, the crews of the Star Destroyers look a bit young. That's because they're the children of the Sith Eternal faithful, raised with a fanatical devotion to the Dark Side and trained since birth for this very specific purpose. The ships were also outfitted with updated automation systems, giving them smaller crew requirements just in case.
So, that's the long answer to how Palpatine's fleet came to be in the factories beneath the surface of Exegol, but the short answer seems to be that some of the wealthiest and most powerful people in the galaxy definitely didn't see the error of their ways after the Emperor's death, and decide to use their influence to enact a horrifying contingency plan.