Exemple of static Instance for a GameManager, SoundManager, etc... in Unity + GameManager version ready for ads

GameManager.cs

#region Author
/////////////////////////////////////////
//   RAPHAËL DAUMAS --> GameManager
//   https://raphdaumas.wixsite.com/portfolio
//   https://github.com/Marsgames
/////////////////////////////////////////
#endregion
using UnityEngine;

[HelpURL("https://gist.github.com/Marsgames/f5808a289f2ee135ac08930937787c68")]
public class GameManager : MonoBehaviour
{
    #region Variables
    [HideInInspector] public static GameManager Instance { get; private set; }
    #endregion Variables

    ///////////////////////////////////////////////////////////

    #region Unity's functions
    private void Awake()
    {
        if (!Instance)
        {
            Instance = this;
        }
        else if (this != Instance)
        {
            Destroy(gameObject);
        }
        
        DontDestroyOnLoad(this);
    }
    
    private void Start() 
    {
        CheckIfSetUp();
    }
    #endregion Unity's functions

    ///////////////////////////////////////////////////////////

    #region Functions
    #endregion Functions

    ///////////////////////////////////////////////////////////

    #region Accessors
    #endregion Accessors
    
    ///////////////////////////////////////////////////////////

    #region Utils
    /// <summary>
    /// Checks if all variables are set up correctly, otherwise close Editor
    /// </summary>
    private void CheckIfSetUp ()
    {
#if UNITY_EDITOR
        bool isSetUp = true;

        if (!randomVariable)
        {
            Debug.LogError($"<b>Random Variable</b> cannot be null in <color=#00f> {name} </color>", gameObject);
            isSetUp = false;
        }

        UnityEditor.EditorApplication.isPlaying = isSetUp;
#endif
    }
    #endregion Utils
}

GameManagerWithAds.cs

#region Author
/////////////////////////////////////////
//   RAPHAËL DAUMAS --> GameManager
//   https://raphdaumas.wixsite.com/portfolio
//   https://github.com/Marsgames
/////////////////////////////////////////
#endregion
using System.Collections;
using UnityEngine;
using UnityEngine.Advertisements;

[HelpURL("https://gist.github.com/Marsgames/c7330e598d7e1ef00bdbad07dc450bdb")]
public class GameManager : MonoBehaviour, IUnityAdsListener
{
    #region Variables
    [HideInInspector] public static GameManager Instance { get; private set; }

    private string adsID;
    private bool adsTestMode = false;
    #endregion Variables

    ///////////////////////////////////////////////////////////

    #region Unity's functions
    private void Awake()
    {
        if (!Instance)
        {
            Instance = this;
        }
        else if (this != Instance)
        {
            Destroy(gameObject);
        }

        DontDestroyOnLoad(this);
    }

    void Start()
    {
        CheckIfSetUp();
    
#if UNITY_IOS
        adsID = ; // IOS Ads ID (found on Unity Dashboard)
#elif UNITY_ANDROID
        adsID = ; // Android Ads ID (found on Unity Dashboard)
#endif

#if UNITY_EDITOR
        adsTestMode = true;
#endif

        Advertisement.Initialize(adsID, adsTestMode);
        ShowAds("Banner");
    }
    #endregion Unity's functions

    ///////////////////////////////////////////////////////////

    #region Functions
    /// <summary>
    /// Show banner when the ad is ready
    /// </summary>
    private IEnumerator ShowBannerWhenReady()
    {
        while (!Advertisement.IsReady("Banner"))
        {
            yield return new WaitForSeconds(0.5f);
        }

        Advertisement.Banner.SetPosition(BannerPosition.BOTTOM_CENTER);
        Advertisement.Banner.Show("Banner");
    }
    #endregion Functions

    ///////////////////////////////////////////////////////////

    #region Ads
    public void ShowAds(string placementID)
    {
        OnUnityAdsReady(placementID);
    }

    public void OnUnityAdsReady(string placementId)
    {
        switch (placementId)
        {
            case "rewardedVideo":
                Advertisement.Show(placementId);
                break;

            case "Banner":
                // PackageManager --> Advertisement --> v3+ requiered
                StartCoroutine(ShowBannerWhenReady());
                break;

            case "video":
                Advertisement.Show(placementId);
                break;

            default:
                Debug.LogWarning($"{placementId} is an unknown ads ID", this);
                break;
        }
    }

    public void OnUnityAdsDidError(string message)
    {
        Debug.LogError(message, this);
    }

    public void OnUnityAdsDidStart(string placementId)
    {
        // Maybe put the game in pause
        Time.timeScale = 0;
    }

    public void OnUnityAdsDidFinish(string placementId, ShowResult showResult)
    {
        switch (placementId)
        {
            case "rewardedVideo":
                // Define conditional logic for each ad completion status:
                if (showResult == ShowResult.Finished)
                {
                    // Action to do when the ads end
                    // Go to next level, give reward, double score...
                }
                else if (showResult == ShowResult.Skipped)
                {
                    // Action to do if the ads is skipped
                    // Go to main menu, go to end menu...
                }
                else if (showResult == ShowResult.Failed)
                {
                    // Action to do if playing ads failed (no internet)
                    // Debug.LogWarning(), go to main menu, go to end menu...
                    Debug.LogWarning("The ad did not finish due to an error");
                }
                break;

            case "banner":
                return;

            case "video":
                // Go to next level, go to main menu...
                break;

            default:
                break;
        }

        // Restore timescale if changed;
        Time.timeScale = 1;
    }
    #endregion Ads

    ///////////////////////////////////////////////////////////

    #region Accessors
    #endregion Accessors
    
    ///////////////////////////////////////////////////////////

    #region Utils
    /// <summary>
    /// Checks if all variables are set up correctly, otherwise close Editor
    /// </summary>
    private void CheckIfSetUp ()
    {
#if UNITY_EDITOR
        bool isSetUp = true;

        if (!randomVariable)
        {
            Debug.LogError($"<b>Random Variable</b> cannot be null in <color=#00f> {name} </color>", gameObject);
            isSetUp = false;
        }

        UnityEditor.EditorApplication.isPlaying = isSetUp;
#endif
    }
    #endregion Utils
}

I wonder if I should use a constructor or not