def add_ad_to_exist_ad_group(
        client: GoogleAdsClient,
        customer_id: str,
        ad_group_id,
        assets_by_ad_group: Dict[str, List[str]],
        headlines: List[str],
        descriptions: List[str],
) -> None:
    # assets_by_ad_group = {
    #     'customers/2309565764/adGroups/135305191091':
    #         [
    #             'customers/2309565764/assets/18099191709',
    #             'customers/2309565764/assets/18099191769',
    #             'customers/2309565764/assets/18099191733',
    #             'customers/2309565764/assets/18099191811'
    #         ]
    # }

    ad_group_ad_service = client.get_service("AdGroupAdService", version=GOOGLE_API_VERSION)
    ad_group_service = client.get_service("AdGroupService")
    ad_group_ad_operations = []

    for ad_group_resource_name, video_assets_resource_names in assets_by_ad_group.items():
        ad_group_ad_operation = client.get_type("AdGroupAdOperation")
        ad_group_ad = ad_group_ad_operation.create
        ad_group_ad.ad_group = ad_group_service.ad_group_path(
            customer_id, ad_group_id
        )
        ad_group_ad.ad.app_ad.headlines.extend(
             [create_ad_text_asset(client, headlines[0]), create_ad_text_asset(client, headlines[1])]
         )
        ad_group_ad.ad.app_ad.descriptions.extend([create_ad_text_asset(client, descriptions)])
        ad_group_ad.status = client.enums.AdGroupAdStatusEnum.ENABLED
        ad_group_ad.ad.app_ad.youtube_videos.extend(
            [
                create_ad_video_asset(client, resource_name)
                for resource_name in video_assets_resource_names
            ]
        )
        ad_group_ad_operations.append(ad_group_ad_operation)

    ad_group_ad_response = ad_group_ad_service.mutate_ad_group_ads(
        customer_id=customer_id, operations=ad_group_ad_operations
    )
    for result in ad_group_ad_response.results:
        print(f'Created ad group ad "{result.resource_name}".')

def create_ad_video_asset(client: GoogleAdsClient, video_resource_name: str) -> Any:
    ad_video_asset = client.get_type('AdVideoAsset', version=GOOGLE_API_VERSION)
    ad_video_asset.asset = video_resource_name
    return ad_video_asset