Hi Sarainia,
yes, you can actually create programs for MainOS. Currently I'm writing the documentation for that on GitHub. To get started, you can follow these instructions: https://github.com/Maingron/MainOS/wiki/Creating-your-first-program#. Like I said, I'm still working at the documentation, so it's not complete yet, but it should still be able to help you getting started.
To save new documents with the notepad, you have to type the path into the input field within the save File-Button (eg C:/users/testfile.txt). Afterwards click on save.
You can also edit already existing files by opening them with the explorer. In this case the path is already set.
I'd actually appreciate if you create a program, since I'm just one "junior" developer working on this project.
If you have any questions, feel free to contact me :) You can decide for a possibility at https://maingron.com/contact