Could not find an answer to my doubts and hopefully somebody can clarify.
I Have created a dummy solution with
You could try to use a shared library project. It compiles against the platform of the referencing application/library, so to speak. That gives you the ability to create class libraries targeting different platforms without the need to duplicate any code, but it may require some #if...
https://blogs.msdn.microsoft.com/dotnet/2014/04/21/sharing-code-across-platforms/