I want to learn how to create apps in iOS, macOS, and iPadOS. But I don’t know where to start. What is different with developing with iOS, macOS, and iPadOS? Where should I start? iOS, macOS, or iPadOS?
You're seemingly approaching this as an end-point, and not as yet another waypoint of what may well be a half-century of working, and ongoing learning.
In IT generally, and with Apple and other major vendors more specifically, you will never be done learning, will never know all you need to know, and will never stop having to learn. Not if you want to keep going in this business. If anything, you'll find you know less about more, as the whole business grows and as computers are entwined into everything.
Much of the technology, and various of the hardware and software vendors, what you have worked on and learned for today will be outdated and outmoded and there'll be work to replace it in five or ten years, maybe in twenty years for the largest installations when the whole factory is rebuilt.
Punched cards and 16-bit computing aren't that long ago. Where we'll all be and what we'll be using in the next ten or twenty years? The biggest and second computing vendors from the 1980s are niche and gone, now.
As for which of the Apple platforms to learn, it doesn't much matter. They're all related. You'll need a Mac for the Xcode tooling and can develop for all of the Apple platforms from there, or you're using an iPad and developing from there with the playgrounds support announced in WWDC 2021. The former costs more, of course.
Find the available course materials that interest you and that you can afford, and find a problem or an app or potential employers that are interesting, and have at. The available materials at the depth you need at a price you can afford is probably the biggest deciding factor in your path, at least for now.
But don't think this learning stuff ever ends.
