I guess it depends on why you’re learning mobile development. If you’re doing it to make your own apps and you want them to be cross platform then I, personally, would use Flutter.

If you want cross-platform apps and you’re looking for a job doing that sort of thing then I’d say that Flutter is extremely niche and I’d look at using JS/React Native. That also give you a bridge to HTML/CSS/JS/React browser development.

If, OTOH, you’re looking for an iOS dev job then you need UIKit. SwiftUI may not even be out of beta stage by EOY and as mentioned in the article most dev projects will need to support UIKit for a year or two or more as they support current version minus one, minus two, etc..

Luck.