Trying to configure parent link and child links in menu I'm trying to create a submenu in the same way shown in the screenshot
module.links.menu.yml:
custom_forms_entity_export_csv.export:
  title: 'Export'
  description: 'Export Menu'
  route_name: custom_forms_entity_export_csv.export
  parent: commerce.admin_commerce

custom_forms_entity_export_csv.export_sales_form:
  title: 'Export Sales'
  description: 'Export Sales to a CSV file.'
  route_name: custom_forms_entity_export_csv.export_sales_form
  parent: commerce.admin_commerce

custom_forms_entity_export_csv.export_menu_form:
  title: 'Export Menu'
  description: 'Export Menu to a CSV file.'
  route_name: custom_forms_entity_export_csv.export_menu_form
  parent: custom_forms_entity_export_csv.export

module.routing.yml
custom_forms_entity_export_csv.export:
  path: '/admin/commerce/export'
  defaults:
    _controller: '\Drupal\system\Controller\SystemController::systemAdminMenuBlockPage'
    _title: 'Commerce'
  requirements:
    _permission: 'access commerce administration pages'

custom_forms_entity_export_csv.export_sales_form:
  path: '/admin/commerce/export/sales'
  defaults:
    _title: 'Sales'
    _form: \Drupal\custom_forms_entity_export_csv\Form\SalesExportCsvForm
  requirements:
    _permission: 'use custom export csv'

custom_forms_entity_export_csv.export_menu_form:
  path: '/admin/commerce/export/menu'
  defaults:
    _title: 'Menu'
    _form: \Drupal\custom_forms_entity_export_csv\Form\MenuExportCsvForm
  requirements:
    _permission: 'use custom export csv'

module.links.task.yml
custom_forms_entity_export_csv.export_sales_form:
  title: 'Download Sales'
  base_route: custom_forms_entity_export_csv.export
  route_name: custom_forms_entity_export_csv.export_sales_form
  weight: 100

custom_forms_entity_export_csv.export_menu_form:
  title: 'Download Menu'
  base_route: custom_forms_entity_export_csv.export
  route_name: custom_forms_entity_export_csv.export_menu_form
  weight: 100

But all I end up with is all three links, Export, Export Sales, and Export Menu as children under the same Commerce menu, like shown here:

What I want is for Export to have an arrow on the right to expand a submenu containing Export Sales and Export Menu.
Your 2nd menu link needs the parent property referencing your first custom_forms_entity_export_csv.export route :
custom_forms_entity_export_csv.export_sales_form:
  title: 'Export Sales'
  description: 'Export Sales to a CSV file.'
  route_name: custom_forms_entity_export_csv.export_sales_form
  parent: custom_forms_entity_export_csv.export