Hello there,
I am an experienced programmer. I can do C/C++/Rust/assembly/Ruby/Perl/Python/ etc.. The language itself is not a barrier.
The barrier to me is that I have never coded a single web or android application. I guess it must be surprising but I am more of a low-level programmer in my job (I develop a compiler backend) and I never really had the opportunity or idea to work on an app.
What would be a good starting point for making an android application?
A quick search got me this: https://google-developer-training.github.io/android-developer-fundamentals-course-concepts-v2/unit-1-get-started/lesson-1-build-your-first-app/1-1-c-your-first-android-app/1-1-c-your-first-android-app.html
Would it be a good starting point?
Side note: my app will not have to interact with any service. If I were to code it as a command-line program, it would not take me more than a day or two. The actual app would involve (for now) no more than a text field, a button, some logic attached to it - the hard part for me being to choose a framework to build it, "upload it" to my phone and use it.
You can build it in Godot and export it to your android. Alternatively, you could build a webpage and access it via your phone browser.
Godot is how I'd do it. I've done something similar before.
Godot is great but probably overkill for what OP wants to do. I would choose something like Flutter instead for a basic app like this.