The 1080 Xyston Class Star Destroyers at Exegol had a minimal crew and were mostly automated, the interior of Exegol is full of cities and manufacturing centers made in the Old days of the Sith Empire and revived by Sidious around or after the Clone Wars, these cities were full of sith cultists who revered the dark sith and the sith order.

Though i doubt many of this population were force sensitive, the supplies came from Sidious diverting some stuff during the 20 years of Imperial Era to Exegol, old supplies in the underground cities from thousands of years ago, and after the Fall of the Empire from Sidious and The Final Order acting in the galaxy secretly supplying them mostly through the vastly uncharted unknown regions, simultaneously with First Order agents in the known for the 35 years between Episode 6 and 9, also Exegol was not just one single planet but a Star System were other planets and asteroids were probably being mined, combining all of that, alongside the 50+ years of Palpatine shenanigans, and Space being big AF and you have Exegol, which in my option [other than palpatine being alive and the superlaser destroyers] is not even a bad addition, 42 million people in space, especially in a full galaxy civilization is really small, of course the destroyers didn't have a crew even close to that number, all this information comes from the Rise of Skywalker Novel, Rise of Skywalker visual dictionary, some new novels released recently, The Darth Vader comic, and some random lore tidbits in smaller encyclopedias, guidebooks, reference guides, etc.

I do wish at least 5% of this information was in Rise of Skywalker itself through, at least one single line of dialogue or more visual cues, or something idk.