Awesome that you want to learn something new :) Search hashnode for similar questions, they will help you understand the differences between using javascript and using Objective-C/Java for app building.
tl;dr is using native langs or something like React native depends on how complex you want the app to be. This is because native langs don't have the same limitations that React native does (and similar frameworks for building apps with javascript).
iOS requires you to buy a license to start developing (or at least last time I checked) so I would definitely start with android or react as they are both free to begin.
Personally, I like android/Java+XML but then again it's been over a year since I developed on it and I never developed with iOS (something that might change by the end of the year lol).
Wish you the best :)
Edit:
The community is strong regardless of which you choose.