If you're really serious about iOS/macOS programming, you might wanna spend a lot of times perusing Apple's documentation. You might wanna use a real Mac computer for this, and spend like $99 (varies between countries) for testing your app in real devices (You can postpone this until you're ready to test/distribute though, because it's only for one year of iOS app developer account)
Also, in XCode, there's a documentation reference, where you can add Apple's docset, so you can look at the documentation in XCode and download sample codes without accessing the internet. The documentations are plenty.
You might wanna start with learning the Objective-C first, because you need to know the language first. It's different than C. Although similar in some parts.
And then you can start move to iOS (cocoa touch) core competencies, that will teach you about the methodology and the basic things you really need to know about iOS applications.
After that you might want to move to programming guides, like View Controller's programming Guide, Core data programming guide, Web Services Programming Guide, etc.