Star Wars has debunked the popular theory from The Rise of Skywalker that Emperor Palpatine was actually Darth Plagueis all along. In Star Wars Episode III - Revenge of the Sith, Palpatine told Anakin Skywalker of his own Sith Master, Darth Plagueis the Wise, who had discovered a way to cheat death using the dark side of the Force.
Star Wars: The Rise of Skywalker revealed that the secret was an ancient Sith power known as "Essence Transfer." This allows a recently-deceased Sith to transfer their spirit to another vessel; a nearby object or person. It's much easier to inhabit someone if they are full of the dark side, explaining why Palpatine wanted Rey to act in rage and hate; she would be less able to resist his baleful influence. This ability appears to radically change the context of the Sith Rule of Two; it raises the possibility that each Sith Lord has possessed the body of his or her Apprentice, explaining why the Sith have been growing in power over the course of millennia, becoming a malignant imbalance in the Force. Thus Palpatine is himself Plagueis, making his account to Anakin Skywalker supremely ironic. The theory was apparently confirmed by a single line of dialogue in Star Wars: The Rise of Skywalker, when Palpatine boldly proclaimed himself to be the embodiment of "all the Sith."
However attractive this theory may be, however, the novelization of Star Wars: The Rise of Skywalker explicitly debunks it. In one key scene, Palpatine remembers how he survived his death:
"falling... down a massive shaft, the betrayal sharp and stinging, a figure high above, black clad and helmeted and shrinking fast. His very own apprentice had turned against him, the way he himself had turned against Plagueis... whose secret to immortality he had stolen.
Plagueis had not acted fast enough in his own moment of death. But Sidious, sensing the flickering light in his apprentice, had been ready for years. So the falling, dying Emperor called on all the dark power of the Force to thrust his consciousness far, far away to a secret place he had been preparing. His body was dead, an empty vessel, long before it found the bottom of the shaft, and his mind jolted to new awareness in a new body - a painful one, a temporary one."
It seems Essence Transfer was indeed the secret of Plagueis' immortality. Of course, that adds another layer of irony and misdirect to Palpatine's words in Star Wars: Episode III - Revenge of the Sith; he tempted Anakin with the dark side as a way to keep Padmé alive, when this power would only have allowed Anakin to save himself from death. All the most compelling lies are half-truths, and Palpatine has always been supremely deceitful.
Amusingly, this transforms Charles Soule's Darth Vader run into setup for Star Wars: The Rise of Skywalker. That revealed Palpatine had continued researching Essence Transfer after he had become Emperor, and that he had obtained the mask of a Sith heretic named Lord Momin. Momin had died in a powerful conflagration of Dark Side energy, and his essence had been trapped within the mask he had worn. To be in the presence of Momin's mask was to fall under the influence of the dark side of the Force, driven to rage and bloodlust, while actually wearing the mask meant you were possessed by his spirit.
The key difference between Palpatine's Essence Transfer and Lord Momin's, however, is the fact Palpatine was able to possess a clone body that was light years away, whereas Momin had possessed his own mask. It's possible the Emperor used a realm known as the World Between Worlds, which he discovered in Star Wars Rebels; that would explain the flare of blue light as Palpatine apparently died, which corresponds with portals to access that mysterious plane of existence.