How can we load a Menu2 from an xml File and show it in a layout?

You can't use Menu2 in a layout.  It's it's own view.  You need to push it for it to be used.

/* Replace this code and use XML menu resource instead */

 var menu = new WatchUi.Menu2({:title=>"My Menu2"});
      
        menu.addItem(
            new MenuItem(
                "Item 1 Label",
                "Item 1 subLabel",
                "itemOneId",
                {}
            )
        );
        menu.addItem(
            new MenuItem(
                "Item 2 Label",
                "Item 2 subLabel",
                "itemTwoId",
                {}
            )
        );
    
        WatchUi.pushView(menu, null, WatchUi.SLIDE_IMMEDIATE);

How can I replace this code by loading an xml menu2 file?

Oh, you're talking about defining a Menu2 in xml, and not using it in another layout.

I'm not sure, as I generally define a menu in the code so it can be more dynamic, based on the state of the app, setting the text ,toggles, etc. 

Unknown said:

The resource documentation says nothing about menus.

You're looking at the wrong section. Have a look here:

  https://developer.garmin.com/connect-iq/programmers-guide/user-interface#menu2xmlresources

Just figured it out by taking a look at the "MenuTest" example project and not the "Menu2Sample".

What I couldn't figure out is that I can create the Instance of the menu by accessing "Rez.Menus.Menuname"  and it is not mentioned anywhere in the documentation. Unfortunately Eclipse didn't show the auto complete option (only after restarting the editor).

WatchUi.pushView(new Rez.Menus.StartSelectionMenu(), null, WatchUi.SLIDE_IMMEDIATE);