What are things that every iOS developer must know/ do to improve their skills?

Certainly one of the things is to keep building stuff. I’ve recently decided to write a small app (I went for a clone app because I don’t want to deal with designing it) and it has really forced me to learn SwiftUI (I’ve only used UIKit professionally), while previously I’d read articles without much reason to hold on to them.

So if you want to learn something, find a project that will force you to learn that thing, but if the purpose is to learn, don’t try to make it an “original idea” or something like that. It would only lead to procrastination. Find an existing tool that makes use of what you want to learn and try to implement it yourself.