unity how to make a coroutine code example

Example 1: unity coroutine

void Start() {
  StartCoroutine("func"); // Start coroutine named "func"
}

IEnumerator func() {
  Debug.Log("Hello");
  yield return new WaitForSecondsRealtime(1); //Wait 1 second
  Debug.Log("World");
}

Example 2: coroutine start unity

IEnumerator Start()
    {
        Debug.Log("Start1");
        yield return new WaitForSeconds(2.5f);
        Debug.Log("Start2");
    }