How to check if the application is running?

UNITY 2D C#

I would like to “do something” when the application is running on a mobile platform. I added the Application.isPlaying function, but this function detects the launch each time I go to the scene where the script is placed, i.e. the script with the Application.isPlaying function has been added to the Menu Scene. When I launch the application, it is detected. However, when I go to another scene and return to the Menu, the script detects it as the next time the application is started.

How to change that?

using System.Collections;
using System.Collections.Generic;
using UnityEngine;
using UnityEngine.UI;


public class WelcomeSystem : MonoBehaviour
{

	public GameObject DD;
    // Start is called before the first frame update
    void Start()
    {
		if ( Application.isPlaying)
		{
            Debug.Log ("AppIsRunning!");
			DD.SetActive(true);
			
		}
        
    }

	public void OnClickUnactive ()

	{
		DD.SetActive(false);
	}

    // Update is called once per frame
    void Update()
    {
        
    }
}

Try putting the first if statement isPlaying in your update? because in this link it says Update changes.