Check out example codes for "billboard canvas unity". It will help you in understanding the concepts better.

Code Example 1

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

public class BillboardFX : MonoBehaviour
{
	public Transform camTransform;

	Quaternion originalRotation;

    void Start()
    {
        originalRotation = transform.rotation;
    }

    void Update()
    {
     	transform.rotation = camTransform.rotation * originalRotation;   
    }
}

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