How to create custom module for admin area in drupal 8
Main question is "how to have a module show up in admin area".



Create your custom module
Create your custom route in the "your_module.routing.yml" - file


Example:

your_module.admin_settings:
  path: '/admin/config/your_module/import'
  defaults:
    _form: '\Drupal\your_module\Form\ImportForm'
    _title: 'Import XYZ'
  requirements:
    _permission: 'administer site configuration'



Define the link in the "your_module.links.menu.yml" - file


Example:

entity.your_module.admin:
  title: 'Import XYZ'
  route_name: your_module.admin_settings
  description: 'Import XYZ'
  parent: system.admin_config_system
  weight: 100


... thats it!
In step 3 this line is the important part:

parent: system.admin_config_system


If you need a menu entry in "structure" you can use:

parent: system.admin_structure


Just read other xyz.links.menu.yml - files to find different parent-path.