Auto-boot your Unity game when entering Play mode

LoadInitialSceneOnEditorPlay.cs

using System;
using System.Collections.Generic;
using UnityEditor;
using UnityEditor.SceneManagement;
using UnityEngine.SceneManagement;

namespace EternalConstruct.Editor.Utils
{
    [InitializeOnLoad]
    public static class LoadInitialSceneOnEditorPlay
    {
        private const string InitialSceneName = @"Bootstrap";
        private const string PreviousScenesPrefsKey = "_PreviousScenes";

        static LoadInitialSceneOnEditorPlay()
        {
            EditorApplication.playModeStateChanged += change =>
            {
                switch (change)
                {
                    case PlayModeStateChange.ExitingEditMode:
                        if (!EditorSceneManager.SaveCurrentModifiedScenesIfUserWantsTo())
                        {
                            EditorApplication.isPlaying = false;
                            return;
                        }
                        
                        var previousScenes = new List<string>();

                        for (var i = 0; i < SceneManager.sceneCount; i++)
                        {
                            previousScenes.Add(SceneManager.GetSceneAt(i).path);
                        }
                        
                        EditorPrefs.SetString(PreviousScenesPrefsKey, string.Join("|", previousScenes));
                        
                        previousScenes.Clear();
                        
                        EditorSceneManager.OpenScene($"/Path/To/Your/Scenes/{InitialSceneName}.unity");
                        break;
                    case PlayModeStateChange.EnteredEditMode:
                        var scenesToRestore = EditorPrefs.GetString(PreviousScenesPrefsKey).Split('|');
                        
                        for (var index = 0; index < scenesToRestore.Length; index++)
                        {
                            var scene = scenesToRestore[index];

                            EditorSceneManager.OpenScene(scene,
                                index == 0 ? OpenSceneMode.Single : OpenSceneMode.Additive);
                        }
                        
                        EditorPrefs.DeleteKey(PreviousScenesPrefsKey);
                        break;
                    case PlayModeStateChange.ExitingPlayMode:
                        break;
                    case PlayModeStateChange.EnteredPlayMode:
                        break;
                    default:
                        throw new ArgumentOutOfRangeException(nameof(change), change, null);
                }
            };
        }
    }
}