How do I create tabs in an admin page? How do I create a tab in the admin page using a custom module?
The files defining the routes, links and task are the following.
cst_core.routing.yml
cst.admin:
  path: '/admin/MyCustomModule'
  defaults:
    _controller: '\Drupal\cst_core\Controller\CstController::content'
    link_id: 'cst.admin'
    _title: 'MyCustomModule Management'
  requirements:
    _permission: 'cst admin'
    
cst.admin_menu:
  path: '/admin/MyCustomModule/menu-mgmt'
  defaults:
    _controller: '\Drupal\cst_core\Controller\CstController::content'
    link_id: 'cst.admin_menu'
    _title: 'MyCustomModule Menu Management'
  requirements:
    _permission: 'cst menu'

cst.admin_menu_menu:
  path: '/admin/MyCustomModule/menu-mgmt/menu'
  defaults:
    _controller: '\Drupal\cst_core\Controller\CstController::content'
    link_id: 'cst.admin_menu_menu'
    _title: 'MyCustomModule Menu Management'
  requirements:
    _permission: 'cst menu'

cst.admin_menu_menu_taba:
  path: '/admin/MyCustomModule/menu-mgmt/menu/taba'
  defaults:
    _controller: '\Drupal\arm_core\Controller\CstController::content'
    link_id: 'cst.admin_menu_menu_taba'
    _title: 'MyCustomModule Menu Management tabs'
  requirements:
    _permission: 'cst menu'

cst_core.links.menu.yml
cst.admin:
title: 'MyCustomModule'
description: 'MyCustomModule System'
parent: system.admin
route_name: cst.admin
weight: -20

cst.admin_menu:
title: 'Menu'
description: 'Menu Management.'
parent: cst.admin
route_name: cst.admin_menu
weight: 0

cst.admin_menu_menu:
title: 'Menu'
description: 'Menus.'
parent: cst.admin_menu
route_name: cst.admin_menu_menu
weight: 0

cst_core.links.task.yml
cst.admin_menu_menu_taba:
route_name: cst.admin_menu_menu_taba
title: 'tab-menu A'
base_route: cst.admin_menu_menu

With this, I should have:
-- MyCustomModule
--- Menu Management
---- Menu
+++++ tab-menu A (as a tab) --&gt; but this didn't happend T_T
something like this:
if I suppose cst.admin_menu_menu_tabA: is your sub-menu A then also add 


  cst_core.links.task.yml for tabs or sub menu tabs in an admin page


then add the following code below

cst.admin_menu_menu_tabA:
  route_name: cst.admin_menu_menu_tabA
  title: 'sub-menu A'
  base_route: cst.admin_menu_menu


proceed so on for the rest of the tabs..

and also add a route for cst.admin_menu_menu_tabA in cst_core.routing.yml