A 404 is expected unless you have a default route for https://api-test.domain.it or configured swagger as the default.
If I do not delete the .Net 6.0 web api file also the index.html file is not found (Error 404)
ASP.NET Core applications use a static file handler for returning static files. The files are located in the wwwroot folder. Add a wwwroot folder to the root of the Web API project and add the static file listener to the application configuration if you want to return static files as a test.