For client on Windows, UWP is .NET Core, and WinForms or WPF currently .NET Framework. Though .NET Core 3.0 is meant to bring the full set of Windows UI stacks to .NET Core https://blogs.msdn.microsoft.com/dotnet/2018/08/08/are-your-...
For Tizen you'd also use .NET Core
For client on macOS or Linux you use Mono
For games you'd probably use Unity (Mono based)
For iOS, tvOS, watchOS, Solaris, Android, Solaris, IBM AIX and i, BSD (FreeBSD, OpenBSD, NetBSD), PlayStation 4, XboxOne, WebAssembly you'd use Mono
Can still use Mono for client on Windows and the ASP.NET Core stack is quite happy running on Mono on the server; but they have different focuses.
If you were building a library you'd target the .NET Standard 2.0 "contract" and it would cover all the runtimes.