After .NET Core 3.0, MS said new features will only be added in Core. And when they reach .NET 5, Standard and Core will be identical, with "Standard" being the library you link to regardless which specific .NET implementation you're running on: desktop, IoT, mobile, etc. I believe the diagram only means that "Standard" covers all of .NET 5, not some subset of Core. And .NET Framework is a separate, maintained legacy library off to the side.
Note, I don't work for Microsoft, this is just my understanding from reading their docs and talking with them about it online and at MS Build.