How to build this dll?

[DearVa]

I want to build a dll to import in Unity, I create a VS project and select .NET Framework.I add the reference of UnityEngine.dll and so on. Then add my c# scripts into it. However, when I build it, the reference is netframework(in dnspy). But your dll reference is mscorlib. Could you please tell me how you build this dll? Is it a unity build dll or a VS build dll?
Thanks a lot!