I am a long-time user of and can recommend Zim as an "enhanced notepad" on desktop. Written in python & GTK. Each note is a separate file in a directory tree. You can access/edit notes with any text editor. Rich markup, tags, attachments, plugins.
pkg_add zim
on OpenBSD, in a similar manner on Linux.
Works on Windows too, but tray icon behavior is less convenient (or I didn't read the docs), and being GTK looks alien.
osx - claimed as supported.
No native mobile app, and "toolkits used to build zim cannot just be deployed on a phone OS,
so basically to build an app would mean a whole new development." but Markor is the recommended Android app for Zim, although you need to manually set file type, they say. I didn't check it out myself.
ios, postmarket, sailfish - dunno
I also tried Feathernotes pkg_add feathernotes
- all data in a single file; and had issues with non-latin encoding.
I also know of Cherrytree, I didn't try it, and it is not ported to OpenBSD.
On 3 major desktops take a look at TreeSheets, being most mind-blowing nested zoomable "notepad". Played with it but haven't used it as it is not ported to OpenBSD, I don't lose my hope... All data in a single file, IIRC.
+1 for Syncthing for synchronization of your notes, projects and everything else