Drupal 8 custom module permissions

If like me you have been wondering how the Drupal 8/9 custom module yml files link together, it goes something like this:

xxxx.routing.yml

free_resources_dashboard:
   path: '/free-resources'
   defaults:
      _controller: '\Drupal\free_resources\Controller\FreeResources::dashboard'
      _title: 'My Resources'
   requirements:
      _permission: 'free resources dashboard'

xxxx.permissions.yml

free resources dashboard:
   title: 'Free Resources Dashboard'
   description: 'Allow access to the free resources dashboard page'

The free resources dashboard: line in the xxxx.permissions.yml file links to the _permissions: 'free resources dashboard' line in the xxxx.routing.yml file. If the _permissions: field does not match any of the defined permissions (for the site?) you will get a permissions error when you try and access your page.