I don't think most apps are like that, though. If it is worth having a desktop app (instead of just, you know, having a web app), than that app is probably relying heavily on native integration. Also, mobile apps and desktop apps are usually quite different (as they should), those are two completely different interfaces.
About web, I'm not sure about RN, but Flutter IMO is so terrible on web, that it's so rare that it would make sense to use it, that my default advice would be that write the web in something different, even if you use Flutter already on mobile and desktop.