Check out example codes for "how to switch scenes when you die in unity 3d". It will help you in understanding the concepts better.

Code Example 1

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

//Load based on name of scene
public class SceneScript {
  public void ChangeScene(string scene = "") {
    SceneManager.LoadScene(sceneName:"scenes Name");
  }
}

Code Example 2

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

public class BallCollision : MonoBehaviour
{
    private void OnCollisionEnter2D(Collision2D collision)
    {
        if (collision.gameObject.name == "Ball Red") {
            SceneManager.LoadScene("Scene 2");
        }
    }
}

Code Example 3

using UnityEngine;
 using System.Collections;
 using UnityEngine.SceneManagement;
 [RequireComponent(typeof(Collider))]
 public class ApplyDamage : MonoBehaviour {
 
     //Drag this class on the object you want to modify player's health!
 
     //This handles subtracting and adding health points to player in a convenient way.
     /*As follows:
      * Tick 'Is this healing player' true or false in the inspector (consequently it will add or subtract health points)
      * Declare amount (on a 1000 scale not on 100!)
      * Add a collider or trigger collider depending on your needs. See below the proper sections for different
      * behaviors!
      * */
 
 
     public bool IsThisHealingPlayer;
     public bool IsThisPickupable;
     public int AmountOfHealth;
     public bool instantDeath;
 
     int health;
     Healthbar healthbar;
 
     void Start(){
 
         healthbar = FindObjectOfType<Healthbar>();
         if (healthbar == null) {
             Debug.LogError("Healthbar class is not found in scene!");
         }
 
         if (IsThisHealingPlayer) 
             health = AmountOfHealth * -1;
         else
             health = AmountOfHealth * 1;
 
     }
 
     //Trigger collider:
     //For zone damage (runs every frame and damages/heals player when inside trigger zone (eg. fire, radiation field, etc)
     void OnTriggerStay(Collider other) {
         if (other.transform.tag == "Player" && !IsThisPickupable) {
             healthbar.SendMessage("ModifyHealth", health, SendMessageOptions.DontRequireReceiver);
         }
     }
 
     //Trigger collider:
     //For picking up medicine boxes or health potions
     void OnTriggerEnter(Collider other){
         if (other.transform.tag == "Player") {
             if (instantDeath) {
                 healthbar.health = 0;
             }
             if (IsThisPickupable && healthbar.health < 1000) {
                 healthbar.health = healthbar.health - health;
                 healthbar.health = Mathf.Clamp(healthbar.health, 0, 1000);
                 Destroy(gameObject);
                 SceneManager.LoadScene("T");
             }
         }
     }    
 
     //Plain collider (eg. sphere for bullet)
     //This is to damage player on impact (rock falling, bullet hitting, etc)
     //Note: Check collision matrix for ControllerColliderHit, also, colliding with character controller is a bit more
     //complex so modify this section according to your needs.
     void OnControllerColliderHit(ControllerColliderHit other) {
         if (other.transform.tag == "Player") {
             healthbar.SendMessage("ModifyHealth", health, SendMessageOptions.DontRequireReceiver);
         }
     }
 
     /* This is to be used if the player is a rigidbody (eg. rollerball)
     void OnCollisionEnter(Collision other) {
         if (other.transform.tag == "Player") {
             healthbar.SendMessage("ModifyHealth", health, SendMessageOptions.DontRequireReceiver);
         }
     }*/
 }

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