Collecting objects?

You have to create some sort of counter. You will want to have it on a seperate script. Could be like this: (Code in C# but js works similar)

Inventory.cs:

 //inside monobehaviour
 
 public int counter;    //The amount of items we already collected
 public int maxItems;    //The number of all items in total
 private bool won;
 
 void Awake(){    //Awake will be called before Start.
     //Initialice the variables
     counter = 0;
     maxItems = 0;
      won = false;
 }
 void Update(){
     if(won == false && counter = maxItems){    //Check if we collected all items
         won = true;    //this will prevent calling the win sequence multiple times.
         Winsequence();    //Put your firework/sound/etc code in this function;
 
     }
 }

On your item script you have to get a reference to your inventory script:

PickUp.cs:

     //inside of monobehaviour again
     private Inventory _inventory; reference to the script from above
     
     void Start(){
         //Find the reference to the script from above
         //The script must be placed on a gameObject with the exact name: InventoryGameObject
         //You have to make sure, that there is only one instance of this gameObject in your scene and it must not be destroyed.
         //You can search UnityAnswers for singleton if this becomes a problem
     
         _inventory = GameObject.Find("InventoryGameObject").GetComponent<Inventory>();
         _inventory.maxItems ++; //Every pick up will increase the maxItems integer by one. At frame 1 maxItems will equal the actual number of items in the scene;
     }
 
 void OnMouseDown(){
     _inventory.counter ++;    //increase the counter by one
     Destroy(this.gameObject);    //You probalby got this already
 }