Inventory system, GetAxis actions? (complex problem)

Hello. i made an inventory system for my game - up to 4 items in hands, one is active. Player can drop/use item, by pressing 1 button the red and yellow item change each other (and so on).

But the code is terrible, as a programmer i feel pain looking at it. So the principle is: Inventory empty object attached to the MainCamera, and 4 position objects attached to the Inventory Object (pic 1).

I use 2 GameObject arrays to store Positions and objects, attached to them (pic 2).

I use RayCast to check if an object is Pickupable(c# script named "Pickupable exists) and then perform my PickUp action. So the first problem i have is using GetAxis in Update. I used a trick, in simple form it looks like this:

         if(Input.GetAxisRaw("Fire1") !=0 && PickUpOn)
         {
             // Code of picking up here...
             PickUpOn = false;
         }
 
         if(Input.GetAxisRaw("Fire1") == 0)
         {
             PickUpOn = true;
         }

Is there a way to perform this in a better way?

The next problem is in Changing items by pressing 1,2,3 buttons. I made this:

     void ChangeItem()
     {
         
         if(Input.GetAxisRaw("First Item") !=0 && FirstItemOn)
         {
             if(InventoryItem[3]!=null)
             ChangePosition(3,0);
             FirstItemOn = false;
         }
 
         if(Input.GetAxisRaw("First Item") == 0)
         {
             FirstItemOn = true;
         }
 
         if(Input.GetAxisRaw("Second Item") !=0 && SecondItemOn)
         {
             if(InventoryItem[2]!=null)
             ChangePosition(2,0);
             SecondItemOn = false;
         }
 
         if(Input.GetAxisRaw("Second Item") == 0)
         {
             SecondItemOn = true;
         }
 
         if(Input.GetAxisRaw("Third Item") !=0 && ThirdItemOn)
         {
             if(InventoryItem[1]!=null)
             ChangePosition(1,0);
             ThirdItemOn = false;
         }
 
         if(Input.GetAxisRaw("Third Item") == 0)
         {
             ThirdItemOn = true;
         }
     }

So GetAxis problem + logical uglyness. Any ideas? ))

And also my ChangePosition method:

     void ChangePosition(int first, int second)//first/second - index in InventoryItem array
     {
         temp = InventoryItem[first];
         InventoryItem[first] = InventoryItem[second];
         InventoryItem[second] = temp;
 
         InventoryItem[first].transform.parent = InventoryPosition[first].transform;
         InventoryItem[first].transform.localPosition = Vector3.zero;
         InventoryItem[second].transform.parent = InventoryPosition[second].transform;
         InventoryItem[second].transform.localPosition = Vector3.zero;
     }