AutoLoadGlobalScene - automatically and additively load a global scene in the Unity editor

AutoLoadGlobalScene.cs

using UnityEngine;
using UnityEngine.SceneManagement;
using UnityEditor;
using UnityEditor.SceneManagement;
using System;

// Automatically load additively a "global" scene when the user opens a scene in the Editor
// Just put this file in your project and EDIT the two settings below!
[InitializeOnLoad]
public class AutoLoadGlobalScene {
    // EDIT these two strings to match your project:
    // 1. The name of your global scene:
    public const string globalSceneName = "_Global";
    // 2. The path to your global scene, relative to the project root:
    public const string globalScenePath = "Assets/_Scenes/_Global.unity";

    static AutoLoadGlobalScene() {
        EditorSceneManager.sceneOpened += LoadGlobalSceneIfNeeded;
    }

    static void LoadGlobalSceneIfNeeded(Scene scene, OpenSceneMode mode) {
        if (mode == OpenSceneMode.Single && scene.name != globalSceneName) {
            Scene global = EditorSceneManager.GetSceneByPath(globalScenePath);
            if (!global.isLoaded) {
                global = EditorSceneManager.OpenScene(globalScenePath, OpenSceneMode.Additive);
            }
            EditorSceneManager.SetActiveScene(scene);
            EditorSceneManager.MoveSceneBefore(global, scene); // make top of Editor hierarchy
        }
    }
}