how to fade away scene for few seconds

You can play around with coroutines and Color.Lerp, changing the image's color by a few seconds and then returning it to normal.

Here is a script for you, use it as a base to implement what you really want:

 using System.Collections;
 using UnityEngine;
 using UnityEngine.UI;
 
 public class FadeManager : MonoBehaviour
 {
     [SerializeField]
     private Color opaqueColor = Color.black;
     [SerializeField]
     private Color transparentColor = Color.clear;
     [SerializeField]
     private Canvas canvas;
     [SerializeField]
     private Image image;
 
     private void Start()
     {
         canvas.enabled = false;
     }
 
     public void Fade(float duration)
     {
         duration *= 0.5f;
 
         if (duration <= 0)
         {
             duration = Time.deltaTime / 2;
         }
 
         StartCoroutine(IFade(duration));
     }
 
     private IEnumerator IFade(float duration)
     {
         canvas.enabled = true;
 
         image.color = opaqueColor;
 
         for (float t = 0.0f; t < 1.0f; t += Time.deltaTime / duration)
         {
             image.color = Color.Lerp(opaqueColor, transparentColor, t);
 
             yield return null;
         }
 
         image.color = transparentColor;
 
         for (float t = 0.0f; t < 1.0f; t += Time.deltaTime / duration)
         {
             image.color = Color.Lerp(transparentColor, opaqueColor, t);
 
             yield return null;
         }
 
         image.color = opaqueColor;
 
         canvas.enabled = false;
     }
 }

I didn't tested the code, but it should work.