I have been an Android engineer since my first job 7 years ago. I learned React and React Native along the way and made myself into making apps for iOS. I really like working on this environment and would like to gear myself up with native iOS dev skills.
I do get to design solutions and review other mobile engineers' design docs and code. I often find myself struggling to review the objective C code and nuances of iOS platform API usage (like animations, background operations, ARkit etc).
Where should I start to get better in iOS native app development?
I want to become better at
Since you work at Amazon and it seems like the goal is to provide more value in the iOS realm there, I highly recommend learning by osmosis and just talking with your iOS peers + asking a lot of questions in code review, system design meetings, etc. This is because:
Reading iOS platform documentations
I've been doing iOS development for around 2 years now for Taro, and one of the main things I've learned is that Apple's documentation is terrible. It's hard to read with few examples, and it's often outdated. If you want to learn iOS, the 3rd party resources are much better. Make sure to look for things in the past 1-2 years though as Apple (unlike Google) doesn't really believe in deprecating APIs: They just rip them out, so they'll stop working.
If you want to get good at iOS development in general, I recommend this other discussion: "Good resources to learn iOS app development?"
My recommendation would be to start with a trusted tutorial or start with pair programming something iOS-related at work.
I know the person behind iOS Academy (see https://www.jointaro.com/perks/) and he's helped thousands of people level up their iOS skills.