I recently installed versions of .net core sdk 3.1 and 5. When using the .net5 version today, I found that I couldn't select the version when I created the project, only version 3.1. So I opened the terminal and entered the command: dotnet --list-sdks to query all my installed versions and found that only 3.1.405[C:\Program Files\dotnet\sdks] is displayed. What is the reason for this? I'm sure I have the sdk for .net5 installed. I got it from this site.

So, you can upgrade the VS 2019 version to Visual Studio 2019 16.11.9 version or Visual Studio 2022, and then in the Visual Studio Installer click the "Modify" button on the installed Visual Studio, in the popup window, go to the Individual components panel, checked the .NET 5.0 runtime option:

Then when create new applications using Visual Studio, you can choose the .Net 5.0 version.