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<?php
// Fetching The data via the container (old variable_get)
$data = \Drupal::config('my_module.settings')->get('my_setting_name');

// Fetching the data via the Settings class
use Drupal\Core\Site\Settings;

$settings = Settings::get('http_client_config');
$data = $settings['my_setting_name'];

// Old variable_set saving the configuration
\Drupal::configFactory()->getEditable('my_module.settings')->set('my_setting_name', $new_val)->save();

// Override the setting in the settings.php file
$config['my_module.settings']['my_setting_name'] = "The data";