My experience is with iPhone (yeah yeah boo Apple).

Most of how I learned was just digging through Apple’s documentation, focusing on one goal at a time. How do I draw stuff to the screen? How do I handle touch inputs? How do I use the built in UI elements? How do I play sounds? How do I get GPS data? Things like that. I’d usually have an idea of a specific mini-project that would make use of a specific new tool.

Note that I already had some programming experience (although it wasn’t much) before I started teaching myself this way.

Here’s Apple’s website: developer.apple.com/develop/

Just start by downloading XCode and playing with one of their sample projects. SpriteKit is particularly easy to get started with and there’s a sample project for it. (I’m assuming you want to make something like a game. If you want to make more of a utility app, look up SwiftUI).

If you aren’t an iPhone user “Apple fanboy”, you can try this: developer.android.com/courses

Also many game engines (e.g. Godot, Unreal, Unity) have support for both iOS and Android.