From what I understand it's like:
It was originally intended to be done digitally through I think ToonBoom by Mercury Filmworks. They had to back out of their contract for both Star and Wander Over Yonder due to having more work than they could realistically be expected to complete: Disney felt that Lion Guard and Penn Zero needed to be animated by Mercury because those shows looked more distinctive, while Star/Wander were seen as comparatively easier to be animated by practically any studio.
So Star was shipped to Toon City in the Philippines, with only 5/13 first-season episodes fully animated by Mercury and the remaining ones in various stages of completion (I think it's pretty likely that the latest ones like Storm the Castle hadn't really begun animation by Mercury at all beyond maybe some keyframes). So Toon City either digitally animated the ones mostly finished, which is why some episodes that season past #5 are clearly tweened but look janky, and hand-drew the ones Mercury had done some work for like keyframes but hadn't begun the actual animation process for before breaking contract. Apparently Toon City does both digital and hand-drawn, and so you can see examples of both from eps #6-13.
Then from S2 on Star was divided between two different South Korean animation studios, maaaybe because a lot of people thought the Toon City animation looked bad. One was Sugarcube, which is actually digital, and the other is Rough Draft Korea, which is hand-drawn and actually had worked on one S1 episode that I suspect Toon City might have produced subpar results for beforehand.
Though the episodes each season are roughly split between those two studios, it's pretty obvious that most of the biggest episodes, the really plot-relevant ones and season finales etc., are given to Rough Draft.
Probably more information than you asked for but yeah. People sometimes assume the entire show must be one or the other when it's pretty much always been a mix of both.