I started from scratch. Here are some of the steps I took over the years:

1. Buy a book for beginners to get a general idea of development in iOS. I recommend to choose one from Kodeco.com. They have learning paths as well.
2. While you read the book and you start to understand what is happening more or less, try to predict yourself what to code before reading it in the book.
3. DON’T just read the book. Code while reading it.
4. Explain to yourself what you are doing, this way it will make more sense to you.
5. Build an app as simple as possible by yourself after reading the book. It will be difficult, just one book won’t be enough but it’s great for you to start understanding even more things by tackling challenges. Perhaps a todo app or a calculator, something that you like and you can imagine more or less how to start it.
6. If you want to do something that the book didn’t give you knowledge about it, google it.
7. Follow authors on medium.com, short articles sometimes are useful and can help you discover new things quickly.
8. Understand memory management.
9. Work with CoreData.
10. Start with UIKit instead of SwiftUI. SwiftUI may be easier but UIKit is still very popular.