Star Wars: The Rise of Skywalker made a lot of mistakes, but the ball was definitely dropped when it came to how Emperor Palpatine's return was handled. It took a tie-in novel to explain some of the movie's biggest plot holes from those parts of the "Skywalker Saga" finale, and even then it still didn't really make much sense!
It was on the ancient Sith planet of Exegol that we first learned Palpatine had cloned himself, and amassed a loyal army of followers. A throwaway moment confirmed that Supreme Leader Snoke was a clone (or one of Palpatine's creations), but some newly revealed concept art elaborates on that.
As you can see, in an earlier version of The Rise of Skywalker, Kylo Ren was set to arrive in a laboratory not all that different to those on Kamino, with aesthetics similar to those we see on the Empire's ships. A second piece of artwork shows the former Ben Solo taking a closer look inside one of those cloning vats, likely because he's seen Snoke's familiar face within.
Not taking Exegol down this route was no bad thing, but it certainly would have been interesting seeing how much Palpatine's return differed in a setting like this.
Check out this Star Wars: The Rise of Skywalker concept art below: