Definitely not a dumb question, I was wondering that too. My first guess for cross-platform desktop apps these days is Electron, and Inkdrop's UI looks a little Electron-y. But it would be really interesting to hear if it's something else.

For the mobile apps, afaik React Native is the most capable cross-platform mobile framework right now. Again, would be really interesting if it's something else (or hardcore if they're both native apps).