It depends. If you want to design a game, I suggest Unity & C#.
If you want to design an app, I used to recommend doing it natively in iOS Swift & Android Kotlin.
These days I recommend using what you know, where you feel comfortable, and what will make you go faster. Some people find a lot of success using React native!
Some others like using non-coding frameworks like BuildBox for games and I think Bubble for apps. I'm not sure tho.
@AppsWithoutCode also is a good resource!