Form definition example (located at sample / src / Form / sampleSettings.php):

namespace Drupal \ sample \ Form;

use Drupal \ Core \ Form \ ConfigFormBase;
use Drupal \ Core \ Form \ FormStateInterface;

/ **
 * Settings / Configuration form for the module
 * /
class sampleSettings extends ConfigFormBase {

  / ** 
   * {@inheritdoc}
   * /
  public function getFormId () {
    return'sample_settings';
  }

  / ** 
   * {@inheritdoc}
   * /
  protected function getEditableConfigNames () {
    return [
      'sample_settings.settings',
    ];
  }

  / ** 
   * {@inheritdoc}
   * /
  public function buildForm (array $ form, FormStateInterface $ form_state) {
    $ config = $ this-> config (sample_settings.settings);

    $ form ['site_name'] = [
      '#type' =>'textfield',
      '# title' => $ this-> t ('Enter the site name'),
      '# required'=> TRUE,
      ' # default_value '=> $ config-> get ('site_name'),
    ];  

    return parent :: buildForm ($ form, $ form_state);
  }

  / ** 
   * {@inheritdoc}
   * /
  public function submitForm (array & $ form, FormStateInterface $ form_state) {
      $ submit_data = $ form_state-> getValues ​​();
    // Retrieve the configuration.
        $ This-> config ('custom_fetch_ftp. settings')-> set ('site_name', $ submit_data ['site_name'])-> save ();

    parent :: submitForm ($ form, $ form_state);
  }
}

In this way we can fetch our configurations anywhere in our Drupal codebase like this

$ config = \ Drupal :: config ('sample_settings.settings');
$ site_name = $ config-> get ('site_name');