The following contains spoilers from Star Wars: Hidden Empire #1, on sale now from Marvel Comics.
One of the most controversial aspects of the Star Wars sequel movies was how Emperor Palpatine returned to life. He was thought to have been destroyed in Return of the Jedi, blown to bits in the Death Star reactor when Darth Vader tossed him in. But The Rise of Skywalker confirmed, for years, the Emperor was indeed running clone projects and inhabiting new bodies in trial experiments.
It subverted the Expanded Universe lore where Palpatine used soul transference to jump into hosts, seeking the right vessel. This target became Rey on the big screen, but luckily, she and Ben Solo would stop this iteration of the villain. Interestingly, though, Star Wars: Hidden Empire #1 (by Charles Soule, Steven Cummings, Victor Olazaba and Guru e-FX) may have revealed exactly what was the science behind Palpatine's rebirth.
Star Wars Never Explained Emperor Palpatine's Mind-Transference
Now, if clones were being made, it'd stand to reason some would be different. It's why one of these 'sons' -- i.e. Rey's father -- fled and ended up having a family. He just didn't want to follow in Palpatine's legacy, which is why some were wondering how the frail, decrepit shell of the old Palpatine in the black robe emerged at the end.
Sure, it was intimidating seeing him on the throne, but there's no explanation how he had the Emperor's pure spirit inside, versus a Supreme Leader Snoke that felt like an aberration. Again, there's no soul transference in this film, with Palpatine and his cult being, well, a hidden empire all this time -- keen on converting the First Order into the Final Order. However, Palpatine may well have found the key prior to ROTJ.
Hidden Empire's Key Could Be the Fermata Cage
Notably, there's a Sith relic introduced into Hidden Empire called the Fermata Cage. After Darth Maul died, Qi'ra kept using Crimson Dawn to destabilize Palpatine's Imperial forces, and in the process, she stole the Fermata Cage. This hourglass-like device is hooked up to the Knights of Ren, using their Dark Side abilities to try to release a Sith Lord trapped inside. Qi'ra reveals it can freeze individual moments, using the Dark Side to suspend people and places in time forever, nodding to the Phantom Zone. Her plan, like Maul's, is to have this lord go after Palpatine, slay him and become a Master. Given that fans know this doesn't come to fruition, there's a chance Palpatine could defeat Qi'ra, steal this device and use his cult to copy his essence.
Thus, all he'd need is to make the perfect body to place his back-up of a soul in when the time comes. Seeing as his 'son' and the Snoke fiasco didn't pan out, Palpatine could have grown impatient and shifted the plan: putting himself in an old form after discovering the anomaly in Rey. This way, he could move into micromanaging the First Order on a hands-on basis, and then, into possessing Rey. Seeing as the Sith Lord Momin stored his spirit in a mask, Palpatine would already have insight into transferring spirits as he owned this item for a bit. Ultimately, something like this would illustrate how smart Palpatine's disciples were in terms of science, understanding how the spirit rather than a physical body could be the key to immortality, aka the lifelong Sith goal.