Just browse back through the Developer pages to find a project or topic that's helpful.
For example, on python you would find about projects like:
https://www.mobileread.com/forums/sh...d.php?t=254214
https://github.com/Mavireck/Python-S...er/tree/master
https://github.com/SheepWizard/kobo-minesweeper
https://github.com/Slalamander/inkBoard
Similarly, you could find rmkit for C++, go-fbink for golang, plato for rust, chroot for anything, etc...
If you want your app to run on your Kobo ereader, your Android phone, AND your linux desktop, check out the portability phenomenon that is KOReader. Download this calculator plugin and prove to yourself it works AS IS on all three platforms:
https://github.com/zwim/calculator.koplugin
https://koreader.rocks/doc/index.html
(I "made" an instant app by copying this philosopher app and changed it to a "ConfuciusSay" joke app, just by changing the text file and a couple of lines. You can make your own, e.g. "BruceLeeSays", "AristotleSays", or whatever!)
https://github.com/cxzx150133/philosopher.koplugin
Note: If your app has simple GUI requirements like this, you could just implement it as a script in KOReader terminal, or invoke from a Telnet/SSH client. (Or if you install Kobo WebPortal, you could invoke script from remote web browser.)
If not too interactive, you could invoke the script from NickelMenu, with the script directly calling the fbink lib, mentioned by previous commenter, for output (install NickelDBus as well if more user interaction required.)
https://www.mobileread.com/forums/sh...d.php?t=299110
Or html output!
https://www.mobileread.com/forums/sh...postcount=1472
* There have been brave souls venturing into NickelHook programming, but I believe that has portability and maintenance challenges to go with the learning curve. It is nice to let the OG gurus do that heavy lifting.
https://www.mobileread.com/forums/sh...d.php?t=340938
https://www.mobileread.com/forums/sh...d.php?t=364903
https://www.mobileread.com/forums/sh...d.php?t=357540