Welcome. Your questions are a little premature.... as they are answered in the basic intro to the API dev kit and will be further answered by exploring all (or at least a lot) of the example add-ons included in the kit download. All cross-platform information is there, although you'll have questions once you have compiled some of the examples on both platforms and explored things. Have you signed up for the developer web site yet?
You can also browse this forum, and search this forum for C# to see that C# is not really an option (certainly for cross-platform, obviously, as it is not an option in XCode when you create the Mac versions).
PS. I should add that UI development for add-ons is platform independent, but uses an ancient Graphisoft text based methodology. You'll see that the same code will compile, including the UI, for bother platforms - although via Visual Studio for Windows and XCode for Mac. Some here, years ago, have posted ways of converting a graphic sketch to that UI file format, but the simple answer is that there is no visual UI designer as in any modern development environment.
One of the forum moderators
AC 26 USA and earlier • macOS Ventura 13.4, MacBook Pro M2 Max 12CPU/30GPU cores, 32GB