C# Code Understanding - Trigonometry + Angles - Object rotating around player and facing mouse direction

    //Problem: I do not understand how this code works and I require an 
       explanation;
     public GameObject player;
     private Vector3 v3Pos;
     private float angle;
     private readonly float distance = 0.16f;
 
     private void Update()
     {
         v3Pos = Input.mousePosition;
         v3Pos.z = (player.transform.position.z - 
        Camera.main.transform.position.z);
         v3Pos = Camera.main.ScreenToWorldPoint(v3Pos);
         v3Pos -= player.transform.position;
         angle = Mathf.Atan2(v3Pos.y, v3Pos.x) * Mathf.Rad2Deg;
         if (angle < 0.0f) { angle += 360.0f; }
         transform.localEulerAngles = new Vector3(0, 0, angle);
 
         float xPos = Mathf.Cos(Mathf.Deg2Rad * angle) * distance;
         float yPos = Mathf.Sin(Mathf.Deg2Rad * angle) * distance;
         transform.localPosition = new Vector3(player.transform.position.x + 
         xPos * 4, player.transform.position.y + yPos * 4, 0);
     }

Hi, I found this code in a video which makes an object (like a gun) rotate around the player and follow the mouse simultaneously but I don't understand how it works... could anyone tell how it works, step by step? also, I don't know where the video is at anymore but I will find it if necessary.