using Microsoft.AspNetCore.Builder;
using Microsoft.AspNetCore.Hosting;
using Microsoft.Extensions.Configuration;
using Microsoft.Extensions.DependencyInjection;
using Microsoft.Extensions.Hosting;
using Microsoft.OpenApi.Models;

namespace API.Foo
{
    public class Startup
    {
        // ... ... ...

        
        public void Configure(IApplicationBuilder app, IWebHostEnvironment env)
        {
            // ... ... ...
			
            //
            // Path must start with "/" (otherwise, exception will be thrown)
            //			
            app.UsePathBase("/api/foo"); // ←―――――― base path for API
			
            app.UseRouting();

            // ... ... ...
        }
    }
}

using Microsoft.AspNetCore.Mvc;

namespace API.Foo.Controllers
{

    [ApiController]
    public abstract class FooBaseApiController : ControllerBase
    {
        //
        // Put common functionalities only
        //
        // Do not put route attribute
        //
    }
}

using Microsoft.AspNetCore.Mvc;
using System.Reflection;

namespace API.Foo.Controllers
{
    [Route("test")] // ←―――――― explicit attribute route for controller
    public class TestController : FooBaseApiController
    {
        [HttpGet("")] // ←―――――― default action
        public string Index()
        {
            return $@"{{ 
			  ""scope"" : ""{ Assembly.GetAssembly(GetType()).GetName().Name}"", 
			  ""controller-name"" : ""{GetType().Name}"", 
			  ""method-name"" : ""{MethodBase.GetCurrentMethod().Name}"" 
			}}";
        }

        [HttpGet("xyz/{id:int?}")] // ←―――――― explicit attribute route for action
        public string Xyz(int? id)
        {
            return $@"{{ 
			  ""scope"" : ""{ Assembly.GetAssembly(GetType()).GetName().Name}"", 
			  ""controller-name"" : ""{GetType().Name}"", 
			  ""method-name"" : ""{MethodBase.GetCurrentMethod().Name}"", ""Id"" : ""{id}"" 
			}}";
        }
    }
}