Even though the Xamarin and/or .NET Core target platforms (Platform APIs) are treated as if they have the same setup, capabilities, and functionalities as a platform-agnostic framework (.NET Standard), each of these target platforms are different from each other. The adaption layer (implementation of .NET Standard) allows us, developers, to treat these platforms in the same way.
Before the unification and standardization of .NET modules, together with shared projects, cross-platform compatibility was maintained by common denominators of implemented functionality on target platforms. In other words, the available APIs on each selected platform made up a profile that determined the subset of functionality that could be used for these platforms. These platform-agnostic projects that were used to implement the application logic were then packaged into...