Load UXML from C# scripts

To build user interface from a UXML template, you must first load the template into a VisualTreeAsset:

var template = EditorGUIUtility.Load("path/to/file.uxml") as VisualTreeAsset;

or more directly :

var template = AssetDatabase.LoadAssetAtPath<VisualTreeAsset>("path/to/file.uxml");

You can then build the visual treeAn object graph, made of lightweight nodes, that holds all the elements in a window or panel. It defines every UI you build with the UI Toolkit.
See in Glossary that this represents and attach it to a parent element:

template.CloneTree(parentElement, slots);

In the statement above, the <UXML> element in the template isn’t translated to a VisualElement. Instead, all of its children are attached to the element specified by parentElement.

Once the template is instantiated, you can retrieve specific elements from the visual elementA node of a visual tree that instantiates or derives from the C# VisualElement class. You can style the look, define the behaviour, and display it on screen as part of the UI. More info
See in Glossary tree with UQuery: Unity’s implementation of JQuery/Linq.

For example, the following code demonstrates how to create a new EditorWindow and load a UXML file as its content:

using UnityEditor;
using UnityEngine;
using UnityEngine.UIElements;
using UnityEditor.UIElements;

public class MyWindow : EditorWindow  {
    [MenuItem ("Window/My Window")]
    public static void  ShowWindow () {
        EditorWindow w = EditorWindow.GetWindow(typeof(MyWindow));

        VisualTreeAsset uiAsset = AssetDatabase.LoadAssetAtPath<VisualTreeAsset>("Assets/MyWindow.uxml");
        VisualElement ui = uiAsset.CloneTree();

        w.rootVisualElement.Add(ui);
    }
}