MouseLook.cs

using System.Collections;
using System.Collections.Generic;
using UnityEngine;

public class MouseLook : MonoBehaviour
{
    float xRotation = 0f;
    private Vector2 turn;
    public float sensitivity = 250f; 

    // Start is called before the first frame update
    void Start()
    {
        Cursor.lockState = CursorLockMode.Locked;
        Cursor.visible = false;

    }

    // Update is called once per frame
    void Update()
    {
        
        if (Input.GetKeyDown(KeyCode.M)) {
            if (Cursor.lockState == CursorLockMode.Locked) {
                Cursor.lockState = CursorLockMode.None;
                Cursor.visible = true;
            } else {
                Cursor.lockState = CursorLockMode.Locked;
                Cursor.visible = false;
            }
        }

        if (Cursor.visible && Cursor.lockState == CursorLockMode.Locked) {
            Cursor.visible = false;
        }

        if (Cursor.visible) {
            return;

        }
        turn.x += Input.GetAxis("Mouse X") * sensitivity * Time.deltaTime;
        turn.y = Input.GetAxis("Mouse Y") * sensitivity * Time.deltaTime;
        xRotation -= turn.y;
        xRotation = Mathf.Clamp(xRotation, -90f, 90f);
        
        transform.localRotation = Quaternion.Euler(0, turn.x, 0);

        Camera.main.transform.localRotation = Quaternion.Euler(xRotation, 0, 0);

    }
}