How do I create tabs in an admin page?
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