When do target frameworks need to match between nuget and consuming project and when not?

Question

There are nuget packages where dlls only exist for one target framework, like this one:

- lib
  - net40
      SolidWorks.Interop.sldworks.dll
      SolidWorks.Interop.swconst.dll
      SolidWorks.Interop.swpublished.dll

No matter which target framework I choose for a consuming project - e.g. net48, netcoreapp3.1, net5.0-windows, net6.0-windows - it will always use the content from the net40 folder.

I recently created a nuget from a project with the above listed four target frameworks, which resulted in the creation of the following folder structure in the nuget package:

- lib
  - net48
      MyDll.dll
  - netcoreapp3.1
      MyDll.dll
  - net5.0-windows7.0
      MyDll.dll
  - net6.0-windows7.0
      MyDll.dll

For compatibility reasons, I must use the net48 version for all my consuming projects targeting net48 or above. So I just removed the netcoreapp3.1, net5.0-windows7.0 and net6.0-windows7.0 folders from my nuget to get

- lib
  - net48
      MyDll.dll

, but then a consuming project using target framework netcoreapp3.1, net5.0-windows or net6.0-windows complains that it can't find the namespace, classes etc..

So in this case, target frameworks between nuget and project do need to match.

When is which? How can I influence this?

My goal is to create a nuget package that contains just one subfolder in lib with dlls inside so that

• no project with target framework below net48 can work with it
• all projects with target framework net48 or newer work with it.