While Star Wars: The Rise of Skywalker offered very little by way of an explanation for the return of Emperor Palpatine, thanks to the official novelization we have official in-canon confirmation that it wasn’t Old Palpy’s decaying corpse we saw in the movie, but rather a clone body inhabited by the spirit of the Sith Master.
Here’s the relevant passage from the novel, which takes place during Kylo Ren’s first meeting with Palpatine at the start of the movie.
All the vials were empty of liquid save one, which was nearly depleted. Kylo peered closer. He’d seen this apparatus before, too, when he’d studied the Clone Wars as a boy. The liquid flowing into the living nightmare before him was fighting a losing battle to sustain the Emperor’s putrid flesh.
“What could you give me?” Kylo asked. Emperor Palpatine lived, after a fashion, and Kylo could feel in his very bones that this clone body sheltered the Emperor’s actual spirit. It was an imperfect vessel, though, unable to contain his immense power. It couldn’t last much longer.
Of course, a myriad of questions still remain, but no doubt answers will continue to trickle through as Lucasfilm continues to try and cobble together a coherent explanation for the Emperor’s return through books, comics and other media, rather than doing so
Star Wars: The Rise of Skywalker sees J.J. Abrams (Star Wars: The Force Awakens) directing a cast that includes Star Wars veterans Daisy Ridley (Rey), Adam Driver (Kylo Ren), John Boyega (Finn), Oscar Isaac (Poe Dameron), Lupita Nyong’o (Maz Kanata), Domhnall Gleeson (General Hux), Kelly Marie Tran (Rose Tico), Joonas Suotamo (Chewbacca), Billie Lourd (Lieutenant Connix), Greg Grunberg (Snap Wexley), Mark Hamill (Luke Skywalker), Anthony Daniels (C-3PO), Billy Dee Williams (Lando Calrissian), Ian McDiarmid (Emperor Palpatine) and Carrie Fisher (Leia Organa) as well as new additions Naomi Ackie (Lady Macbeth), Richard E. Grant (Logan), Dominic Monaghan (Lost), and Keri Russell (The Americans).