2. The response from Google Directions API may contain alternate routes.
In this example, only the main route is used.

3.1. Unfortunately, I did not understand this problem.
Please rephrase this in more detail.

3.2. Most likely the problem is that you are using the wrong defaultZoom.
The latest versions of Online Maps have a much easier way:

marker3D.sizeType = OnlineMapsMarker3D.SizeType.realWorld;
marker3D.scale = SOME_VALUE;

    should i delete all the parts like :
       foreach (OnlineMapsGoogleDirectionsResult.Leg leg in result.routes[0].legs)
           {
               foreach (OnlineMapsGoogleDirectionsResult.Step step in leg.steps)
               {
                   Debug.Log(step.string_instructions);
               }
          }

or just destroy : result.routes[0]
or its is else ?

---------------------------
2-   what is the index in (result.routes[0]) ?

3- when i used and set a 3D Marker .. there is a 2 problems
1- the marker "Prefab" moved and change its location by moving the map with the mouse cursor and by zooming .its not
    sensitive like the 2D Marker ?
2- i used your function to scale the 3D Marker when zooming . but the result wasn't ok ?

your Function :
using UnityEngine;

public class TestScale3DMarkers : MonoBehaviour
{
    public float defaultZoom = 3;
    public GameObject prefab;
    private OnlineMapsMarker3D marker;

    private void Start ()
    {
        marker = OnlineMapsMarker3DManager.CreateItem(OnlineMaps.instance.position, prefab);

        OnlineMaps.instance.OnChangeZoom += OnChangeZoom;
        OnChangeZoom();
    }

    private void OnChangeZoom()
    {
        marker.scale = Mathf.Pow(2, OnlineMaps.instance.floatZoom - defaultZoom);
    }
}

This works differently, and if you just add PhotonView it will not work correctly.
How to use Photon PUN2 for markers:
https://forum.infinity-code.com/viewtopic.php?id=936