How to add visual elements at runtime using unity's new UI Toolkit/UI Builder

I want to add as many visual elements or buttons using UI toolkit, as I have nodes or skills in my tech tree, problem I am facing is how do I add onto the UXML file or the style sheet at runtime .
any help will be appreciated.

Start with this:

TestUiController.cs

public class TestUiController : MonoBehaviour
{
	[SerializeField] UIDocument _UIDocument = null;
	void OnEnable ()
	{
		var ROOT = _UIDocument.rootVisualElement;
		
		var label1 = new Label( "Hell-o-world" );
		ROOT.Add( label1 );
  
		var button1 = new Button( () => {
			Debug.Log("Button clicked, mission accomplished! :T");
		} );
		button1.AddToClassList( "uss class name for more specific style" );
		ROOT.Add( button1 );
	}
}

to add a button into a specific visual element

// note: element names are displayed in ui Builder with a "#" prefix
var specificElement = ROOT.Q<VisualElement>( "element-name" );
if( specificElement!=null )
{
    specificElement.Add( new Label("found it!") );
}
else Debug.Log( "element not found" );

is it possible to add a button uxml prefab into the document?

// make sure this file exists: (...)/Resources/UI/myButton.uxml
var myButtonUxml = Resources.Load<VisualTreeAsset>("UI/myButton");
var myButtonIntance = myButtonUxml.CloneTree();
ROOT.Add( myButtonIntance);