画面、パネル、そして GameManager

using UnityEngine;
using UnityEngine.SceneManagement;

public class IntroScreen : MonoBehaviour
{
    public void LoadArena()
    {
        SceneManager.LoadScene("Arena");
    }
}

using UnityEngine;
using UnityEngine.SceneManagement;

// Panels, not screens: both of these live inside Arena.unity.
public enum GamePanel { Hud, Pause }

public class GameManager : MonoBehaviour
{
    // The whole arena reaches the manager through this one field.
    public static GameManager I;

    void Awake()
    {
        I = this;
    }
}

[Header("Panels")]
public GameObject hudPanel;
public GameObject pausePanel;

public GamePanel current { get; private set; }

void Start()
{
    ShowPanel(GamePanel.Hud);   // the arena starts playable
}

public void ShowPanel(GamePanel next)
{
    current = next;
    hudPanel.SetActive(next == GamePanel.Hud);
    pausePanel.SetActive(next == GamePanel.Pause);

    // A pause panel that does not stop time is just a picture.
    Time.timeScale = next == GamePanel.Pause ? 0f : 1f;
}

void Update()
{
    if (Input.GetKeyDown(KeyCode.Escape))
        ShowPanel(current == GamePanel.Pause ? GamePanel.Hud : GamePanel.Pause);
}

// Leaving the arena is a screen change, so it goes through SceneManager.
public void BackToIntro()
{
    Time.timeScale = 1f;
    SceneManager.LoadScene("Intro");
}

[Header("Sound")]
public AudioSource musicSource;   // tick Loop on this one
public AudioSource sfxSource;
public AudioClip arenaMusic;
public AudioClip clickClip;

public void PlayMusic(AudioClip track)
{
    // Asking for the track that is already playing must not restart it.
    if (musicSource.clip == track && musicSource.isPlaying) return;

    musicSource.clip = track;
    musicSource.Play();
}

public void PlaySfx(AudioClip clip)
{
    if (clip != null) sfxSource.PlayOneShot(clip);
}