How could the Death Star still be operational in The Rise of Skywalker?

Any idea how that door mechanism is still operational and what is powering it even after 31 in-universe years since the station was destroyed in Return of The Jedi?

This scene is like the scene in the Empire Strikes Back where Luke fights Vader in the cave on Dagobah. As we know that Rey didn't actually fight herself just like Luke did not actually fight Vader in the cave, the door opening/closing on it's own is most likely part of her vision/hallucination. In reality the door may have been open the whole time.

It apparently had its own separated power and (genetic?) recognition system.

Rey crept along the shadowy walls, where she hoped the floor was better supported, and came to a door. It had a complicated access mechanism that marked it as valuable and significant. Maybe a vault. What she sought was assuredly inside.

She could get through this lock eventually. The Death Star had been dormant here for decades. She probably just needed to muscle it—

Rey lifted a hand as if to push, but before she made contact, something clicked and the door whooshed open. As if it still had a power source. Like she’d been recognized.

_{^{Rise of Skywalker: Expanded Edition}}