Class to add all scenes in a Unity project into build settings. This is simpler than adding them manually one by one using UI.

UniyGameScenesInBulk.cs

#if UNITY_EDITOR

using System;
using System.Collections.Generic;
using System.IO;
using System.Linq;
using System.Reflection;
using System.Text;
using System.ComponentModel;
using System.Diagnostics;
using System.Text.RegularExpressions;
using UnityEngine;
using UnityEditor;

namespace Twelve
{
    public class UniyGameScenesInBulk : EditorWindow
    {
        [MenuItem("Tools/Twelve/Add Game Scenes", false, 0)]
        public static void AddTwelveScenes()
        {
            List<EditorBuildSettingsScene> editorBuildSettingsScenes = new List<EditorBuildSettingsScene>(EditorBuildSettings.scenes);
            var sceneFiles = GetScenesInFolder("Game");
            foreach (var sceneFile in sceneFiles) {
                AddOrEnableSceneToSettings(editorBuildSettingsScenes, sceneFile);
            }
            EditorBuildSettings.scenes = editorBuildSettingsScenes.ToArray();
        }

        static void AddOrEnableSceneToSettings(List<EditorBuildSettingsScene> scenes, string path) {
            var existing = scenes.Find(s => s.path == path);
            if (existing == null) {
                EditorBuildSettingsScene sceneToAdd = new EditorBuildSettingsScene(path, true);
                scenes.Add(sceneToAdd);
                UnityEngine.Debug.Log($"Scene '{path}' was added.");
            }
            else if (!existing.enabled) {
                existing.enabled = true;
                UnityEngine.Debug.Log($"Scene '{path}' was enabled.");
            }
        }

        static List<string> GetScenesInFolder(string scenesFolder) {
            string assetsFolder = Application.dataPath;
            string fullPath = $"{assetsFolder}/{scenesFolder}";
            string findCmd = $"find {fullPath} -name '*.unity'";
            var outputLines = new List<string>(ExecuteProcessTerminal(findCmd).Split('\n'));
            var sceneFiles = outputLines.Select(s => s.Replace($"{assetsFolder}", "Assets")).ToList();
            var notEmptySceneFiles = sceneFiles.FindAll(f => f.Trim() != "");
            return notEmptySceneFiles;
        }

        static string ExecuteProcessTerminal(string argument)
        {
            try
            {
                // UnityEngine.Debug.Log("============== Start Executing [" + argument + "] ===============");
                ProcessStartInfo startInfo = new ProcessStartInfo()
                {
                    FileName = "/bin/bash",
                    UseShellExecute = false,
                    RedirectStandardError = true,
                    RedirectStandardInput = true,
                    RedirectStandardOutput = true,
                    CreateNoWindow = true,
                    Arguments = " -c \"" + argument + " \""
                };
                Process myProcess = new Process
                {
                    StartInfo = startInfo
                };
                myProcess.Start();
                string output = myProcess.StandardOutput.ReadToEnd();
                // UnityEngine.Debug.Log(output); // GetStringResult
                myProcess.WaitForExit();
                // UnityEngine.Debug.Log("============== End ===============");
    
                return output;
            }
            catch (Exception e)
            {
                UnityEngine.Debug.Log(e);
                return "";
            }
        }
    }
}

#endif