how to assign different Gameobject to a single variable during run time using c#?

Supposing you are working with 3D objects

1. Add an EventSystem in your scene (GameObject > UI > EventSystem)

2. Attach a Physics Raycaster to the event system

3. Create an empty with the following ClickTargetsManager script:

 using UnityEngine;
 
 public class ClickTargetsManager : MonoBehaviour
 {
     public GameObject CurrentTarget;
 
     private void Start()
     {
         ClickTarget[] targets = FindObjectsOfType<ClickTarget>();
         for ( int i = 0 ; i < targets.Length ; i++ )
         {
             targets[i].OnClicked += OnTargetClicked;
         }
     }
 
     private void OnTargetClicked( ClickTarget target )
     {
         CurrentTarget = target.gameObject;
     }
 }

1. Attach the following ClickTarget script to the objects that must be clicked. Don't forget to add a Collider to each of them before

 using UnityEngine;
 using UnityEngine.EventSystems;
 
 [RequireComponent(typeof(Collider))]
 public class ClickTarget : MonoBehaviour, IPointerClickHandler
 {
     public System.Action<ClickTarget> OnClicked;
 
     public void OnPointerClick( PointerEventData eventData )
     {
         if ( OnClicked != null )
             OnClicked.Invoke( this );
     }
 }