I think you might be slightly confused; [WebMethod] isn't a type of file, it's an attribute you apply to methods in your service to mark them as available for calling remotely. Once you've added the attribute to a method, it will appear in the list of methods exported in the service's webservice definition

using System;
using System.Web;
using System.Web.Services;
[WebService(Namespace = "http://tempuri.org/")]
[WebServiceBinding(ConformsTo = WsiProfiles.BasicProfile1_1)]

public class Service : System.Web.Services.WebService
{
    public Service ()
    {

    }


    [WebMethod]
    public string GetTime()
    {
        return DateTime.Now.ToString("HH:mm:ss");
    }

    public string GetDate()
    {
        return DateTime.Today.ToString("yyyy-MM-dd");
    }
}

If you ran this you'd see:

GetTime, but no GetDate, because GetDate isn't marked as a webmethod

ASMX is very old - it's simple, but very old.. It's increasingly rare to even find people creating XML based webservices (alas) these days, but if they do they probably use WCF, not ASMX

I would recommend you switch to netcore API; the templates work out of the box - you get a Weather Forecast one I think - you can just create a new project, hit play and it works. You can interact with it using e.g. Postman or Insomnia. You can add new operations, it does support parameterized queries to SQL, you don't need EF.. And it'll be JSON based, which is pretty much the direction the modern web has gone in

I solved this problem by opening a new project as: ASP.NET Empty Web Site.
I added it a new item as "Web Service (ASMX)" which created a WebService.asmx file and also a WebService.cs file inside a App_Code folder.

The ASP.NET Web Application I opened before in this question had build in a WebMethod inside ASMX file I added,
as I shown here -

WebService1.asmx

Public Class WebService1
Inherits System.Web.Services.WebService

<WebMethod()> _
Public Function HelloWorld() As String
    Return "Hello World"
End Function

End Class

Which I'm not familiar with this convention of how to implement webMethods in it.
If you know how to write <WebMethod()> _ this way I'll be grateful for your answer :)

Thanks