This diagram illustrates the roadmap for becoming an iOS developer. I updated it to reflect recent changes in 2021. I have tried to collect the most important technical skills that a progressing iOS developer can learn, in a logical order, to succeed more and more.
Without any doubt, Swift is the main programming language for developing new applications for Apple developers, but there are many apps on the App store that have been developed using Objective-C. Many of these apps are serving millions of users worldwide. So to build your career, you probably need to be able to read or even write Objective-C codes.