Camera movement and gun follow in unity

 using System.Collections;
 using System.Collections.Generic;
 using UnityEngine;
 using UnityEngine.EventSystems;
 
 public class MouseComponent : MonoBehaviour
 {
     public float MouseSensitivity = 500f;
 
     public Transform PlayerBody;
 
     float Xrotation = 0f;
 
     //Start is called before the first frame update
     void Start()
     {
         Cursor.lockState = CursorLockMode.Locked;
     }
     
     // Update is called once per frame
     public void Update()
     {
         float MouseX = Input.GetAxis("Mouse X") * MouseSensitivity * Time.deltaTime;
         float MouseY = Input.GetAxis("Mouse Y") * MouseSensitivity * Time.deltaTime;
 
         Xrotation -= MouseY;
         Xrotation = Mathf.Clamp(Xrotation, -90f, 90f);
 
         transform.localRotation = Quaternion.Euler(Xrotation, 0f, 0f);
 
         PlayerBody.Rotate(Vector3.up * MouseX);
     }
 
 }

Attach script to camera and set the playerbody as the gameobject that your character controller is sitting under