As I understand it, .NET Standard is a specification, not an implementation. In other words, it is a list of APIs and they are implemented by different .NET flavors such as .NET Framework, .NET Core, Mono, etc.
And as I understand it, NuGet packages contain implementations for specific .NET flavors. So a NuGut package may contains DLLs for .NET Framework and .NET Core.
So what is happening when a NuGet package is created for .NET Standard? What kind of DLL does it contain?