I know android java, kotlin.. So for iOs development what should I learn?
1.Flutter
2.React native
3.Swift
4.Objective C

Which and why?
I would prefer the one with lowest learning curve.

Swift. It's the current iOS standard for native development.

@SortOfTested how about react native?

@populah
That's not really iOS development. It's ok for early stage mvp, but will need to be replaced at any significant stage of maturity or technical complexity.

@SortOfTested in context of apps that just consume rest apis.. Not any hardware if device

@populah
If you want to use React Native, just use it.

If you don't have any native requirements and if you know some web development then you can go for Ionic framework however it comes with some trade offs. Learning curve is very low however it is not good for native requirements.

Ionic/cordova/phonegap if you dont actually wanna release the app

RN if you want a somewhat working but not good app

Swift if you want an actual app

Flutter if you hate yourself and every night you go to sleep wishing you wouldnt wake up the next day

@k33da I dont think learning curve of Ionic is low if you have web development experience.

Swift, currently there are two flavours for it on IOS, Standard Swift development and Swift UI. Swift in itself is(at least to me) a nice language, not as simple as they want to sell it out to be, but definitely not a bad language, its got some interesting ideas and reads nicely if you write idiomatic code. Also, ios development is not nearly as complex as Android, I learned Android first and then ios, but back then I had to use objective c for what I was doing. Swift is a nice evolution to the way apps are made for Apple systems. This is all personal preference of course, but I prefer bitting the bullet and goin for the native implementation on both systems.