>> in a hypothetical case when I would want to make one, I’m pretty confident it would take me a week or two to learn these things.

The thought might be, how long would it take you to make a great one? An 'expert' might be able to create it by the time someone else has started figuring out lower to mid-level concepts.

The principal doesn't have to make a "great" android app: the principal has to correctly train and marshall their engineers to create a "great" android app. I think you might be stuck in the narrative of the "solo contributor genius". In reality, the vast majority of great products require teams.

The low-level concepts are exactly the same across all modern platforms. Specifically, GUI on all platforms is made of bitmaps (in modern world often encoded into jpeg/png/astc/etc.), vector graphics (polygons, quadratic and cubic Bezier), and font glyphs (same Bezier as in vector graphics + typesetting). These 3 things were invented many decades ago, and stayed pretty much the same over these decades.

Mid.level concepts are more diverse, but not dramatically so. All popular GUI libraries are object oriented, web included. MVC on iOS does have some differences compared to MVVM on Windows, but conceptually they’re still very similar.

It’s funny, your comment highlights exactly why experience is important. You don’t know what you don’t know about Android development. You have a set of very basic assumptions about how the API works, and have no idea what the challenges of Android development are. Sure, you could figure out how to build a basic app quickly, but you probably wouldn’t do a good job building the foundation of a large app being worked on by dozens or hundreds of engineers.

> You don’t know what you don’t know about Android development

What’s that? If you gonna mention Java shenanigans like memory management and garbage collector, .NET is fairly close. Mobile-specific things like multi-touch UX and power management are shared by all battery-powered devices. Many kernel-specific things are shared with all Linuxes including desktop, server, and embedded.

> you probably wouldn’t do a good job building the foundation of a large app

I probably will. I already did a few times in my career. When I found that iOS development job a decade ago, I had zero prior experience with the platform. I had many years of prior experience developing stuff for Windows, game consoles, and some older mobile platforms.

You’re right. You’ve pretty much covered all the bases of Android development.