(UnityEngine) Short and simple camera orbit script with zoom.

MouseOrbit.cs

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

public class MouseOrbit : MonoBehaviour
{
    public float sensivity;

    // Update is called once per frame
    void Update()
    {
        transform.Rotate(0, Input.GetAxis("Mouse X") * sensivity, 0, Space.World);
        transform.Rotate(-Input.GetAxis("Mouse Y") * sensivity, 0, 0, Space.Self);
        transform.GetChild(0).Translate(0, 0, Input.mouseScrollDelta.y);
    }
}