countdown timer acivation

You can achieve the countdown to run on every scene by creating a script like the one below and then just assign it on a gameobject on the screen (if the script it's not running by itself)

 void Start()
 {
  StartCoroutine(Countdown);
 }
 IEnumerator Countdown()
 {
  //set wait time
   yield return new WaitForSeconds(1);
  //do something
 }