So Edna either stores notes in localStorage or in a directory of your choosing on disk.

In Edna you can right-click for context menu to switch between localStorage and disk.

If you ask: "how do the browser APIs work", you can look at https://github.com/kjk/edna/blob/main/src/fileutil.js

Basically, there's `window.showDirectoryPicker()` to ask user for permission to access directory (either read only or read write). And then using that directory handle you can read list of files, read / write files or create new files.