Start here
Tap a card. The words overlap on purpose. Do not hunt for a perfect border between them.
In plain words
An app is a program with buttons.
WhatsApp is an app. A calculator is an app. hello.py is a program, but it is not an app yet. It has no home-screen icon and no tap targets. That is fine. Useful work can look like a dark screen.
Pick a shape
What an APK is
The boxed product made from a recipe. You write the recipe (code). A build step packs it into an APK. Android installs from that box.
The APK is not the running app. It is the install file. F-Droid itself arrived as an APK. Later, if you build a tiny Hello app, you will hold another APK that you made.
A little deeper
Why Android asks before installing one
Anyone can wrap a program in an APK. The question is whether you trust the cook. Official stores check some of that. A random site does not.
Signing, in one kitchen sentence
The box is sealed with a mark that says who packed it. Updates should carry the same mark. You do not need to make that mark in Stage 3.
Next step
An APK has to come from somewhere. The next room is the three doors software uses to reach the phone.
Next step
Two stores. One of them is how Termux should arrive.