Rotate around the Y axis of another object

Unity Engine
1 
using System.Collections;
using System.Collections.Generic;
using UnityEngine;

public class RotateAroundTestScript : MonoBehaviour
{
    public Transform pivot;

    private Vector2 mouseStartPos;
    private Vector2 curMousePos;

    public float x = 0;
    public float y = 0;
    public float z = 0;

    // Start is called before the first frame update
    void Start()
    {
       
    }

    // Update is called once per frame
    void Update()
    {
        MouseRotation();
    }

    private void MouseRotation()
    {
        mouseStartPos = curMousePos;
        curMousePos = Input.mousePosition;
        Vector2 delta = curMousePos - mouseStartPos;
        Vector3 rotAxis = new Vector3(x, y, z);

        transform.RotateAround(pivot.position, rotAxis, -delta.x);
    }
}

I’m learning to use transform.RotateAround(). I’ve got it working the way I want to for the most part. but I’m struggling a bit with the Vector3 axis. I can rotate around something using the global x, y or z, but I want to rotate around an axis on another object.

for example: I want to rotate the cube around the Y axis of the sphere
5085302--500663--rotate.JPG
What API do I use to return the y axis of a game object in Vector3 format? Is that even possible?

2

This should do the job:

        transform.RotateAround(pivot.position, pivot.up, -delta.x);

This means you are rotating your transform arround the pivot position and the pivot y axis