Software _is_ proprietary, ie. closed source. I fully understand that this might be off-putting for someone.
Almost all plugins accessible from the app are OSS, though, and Obsidian has a process of checking them and some kind of control, which I don't know details about.
Obsidian just looks at the folder you specified and all files created with it are just plain text. You can work in notepad all you want and use the software to view the rendered markdown.
