run the command "dotnet --list-sdk" in cmd window, the following msg shows,
5.0.404 [C:\Program Files\dotnet\sdk]
6.0.101 [C:\Program Files\dotnet\sdk]
which means .net core 3.1 sdk not installed.
but indeed I can create and make .net core 3.1 projects using VS2019, even using VSCode. Why .net core 3.1 apps can be created without 3.1 SDK?