I am an experienced programmer. I can do C/C++/Rust/assembly/Ruby/Perl/Python/ etc… The language itself is not a barrier.

Well, first of all, don't try to use any of those languages and recognise that the language is a barrier. Choosing the right tool for the job is critical. Those are great languages... but as far as I know there are precisely zero good user interface frameworks available in those languages.

Just like a good function starts by picking a good name and argument list, a good user interface has to start with a good user interface design. Unfortunately user interfaces are complex beasts and it's virtually impossible to get them right the first time. You absolutely must pick a user interface tool/language/etc which allows you to make major changes (including scrapping the whole thing and stating over) in a short amount of time (minutes, preferably).

The best user interface languages are declarative ones. You should be describing the structure of your interface, largely ignoring the functionality - that's something which can either be done for you by the framework or done yourself as a completely separate task, in a different file, maybe even a different git repository, and probably a different programming language.

It should be possible to get a rough interactive version of your app up and running very quickly, so you can test it, learn what works/doesn't work, show it to other people, and you need to be able to rewrite entire sections of the interface by simply rewriting two or three lines of source code.

I recommend HTML/CSS as a good starting point. After you've got your head around that first (it won't take long, it's relatively simple). After that look into more modern tools like React Native. Learn to crawl before trying to walk.

The article you linked to is just wrong. It suggests this process:

1. Define a layout for each screen that has UI elements.
2. Create source code for all of the app's components.
3. Build and run the app on real and virtual devices.
4. Test and debug the app's logic and UI.
5. Publish the app

Step 4 needs to be tightly integrated into Step 1. Start working on step 2 after you have finished step 4 (and then, after you've done steps 2 and 3, you will need to repeat step 4).

I encourage you to read less articles, they're often giving really bad advice and without experience it's impossible to know which ones are good advice. Instead pay for ChatGPT Plus and just ask it questions. "How do I make a button in HTML/CSS" or "how do I make it execute code when the user clicks it" or "how can I deploy a HTML/CSS/JavaScript app on Android".