@eeeee Probably local to the user computer 🙂
You can generally use non-http URIs for launching different applications. If you were ever redirected from a desktop or mobile app to a browser to log in you've probably seen this in action. Windows actually uses some internally and usually transparently for the user (for example, to abuse their monopoly, MS started using microsoft-edge: scheme instead of https: in some links in Windows to only allow opening them in Edge. They didn't have to build some highly custom mechanism, just restrict other apps from registering this scheme), but usually can be just registered by applications you install. What they do also depends on the application - for example, I think calculator: only launches the calculator app (or at least the obvious way to write math doesn't work), but others can launch specific actions and even pass some information (for example, authentication token for the web login use case I mentioned). For example Spotify allows linking to artists, playlists, albums etc. via spotify: scheme and steam supports doing a ton of things via URI, including launching and even installing/uninstalling games.
All you need is an <a> tag with the right href= set. So yeah, you can put that kind of a link in a widget, but if they wanted to have it be an action under a post, especially if it was supposed to include some information from the post, it wouldn't be that simple.
(side note: NodeBB doesn't allow links using non-standard schemes in markdown, so you can't just put something like this in a post or signature)