how to add time delay

If you're wanting to just add a delay within the current scene, you can use a coroutine for that. You would place your lines 22-28 (though the line numbers are off because the code didn't get formatted properly at the beginning) into a separate function that returns and IEnumerator, and then call that function using StartCoroutine(). Inside of that function you can use return yield WaitForSeconds() to wait before executing the scene-changing code. Here's what that would look like in your code:

 using System.Collections;
 using System.Collections.Generic;
 using UnityEngine;
 using UnityEngine.SceneManagement; /* SUBSCRIBING TO MY YOUTUBE CHANNEL: 'VIN CODES' WILL HELP WITH MORE VIDEOS AND CODE SHARING IN THE FUTURE :) THANK YOU */
 
 public class MoveToNextLevel : MonoBehaviour
 {
     public int nextSceneLoad;
 
     // Start is called before the first frame update
     void Start()
     {
         nextSceneLoad = SceneManager.GetActiveScene().buildIndex + 1;
     }
 
     public void OnTriggerEnter(Collider other)
     {
         if (other.gameObject.tag == "Player")
         {
             if (SceneManager.GetActiveScene().buildIndex == 31) /* < Change this int value to whatever your
                                                                 last level build index is on your
                                                                 build settings */
             {
                 Debug.Log("You Completed ALL Levelws");
 
                 //Show Win Screen or Somethin.
             }
             else
             {
                 StartCoroutine(ChangeScenes());
             }
         }
     }
 
     IEnumerator ChangeScenes()
     {
         yield return new WaitForSeconds(5.0f);
         //Move to next level
         SceneManager.LoadScene(nextSceneLoad);
         //Setting Int for Index
         if (nextSceneLoad > PlayerPrefs.GetInt("levelAt"))
         {
             PlayerPrefs.SetInt("levelAt", nextSceneLoad);
         }
     }
 }

You can learn more about coroutines in Unity here: https://docs.unity3d.com/Manual/Coroutines.html