I have the same issue. Using NuGet was recommended by Bootstrap: https://getbootstrap.com/docs/5.1/getting-started/download/
Those files in the explorer are links to the actual files which can be found in C:\Users\< user >\.nuget\packages\bootstrap\5.1.3\contentFiles\any\any\wwwroot\css\ and \js\
However, they are not visible in the project wwwroot directory and I get a 404 error.
I ended up just skipping this all together and using the CDN method.