SiteSettingsForm

module_name.links.menu.yml

# register menu link
module_name.settings:
  title: 'Site Config'
  description: 'Manage Global Configs'
  parent: system.admin_config_system
  route_name: site_global_config.settings
  weight: 100

module_name.routing.yml

# use unique name for route
site_global_config.settings:
  path: '/admin/config/system/site-configs'
  defaults:
    _form: '\Drupal\module_name\Form\SiteSettingsForm'
    _title: 'Site Configs'
  requirements:
    _permission: 'administer site configuration'

SiteSettingsForm.php

<?php

/**
 * @file
 * Contains \Drupal\module_name\SiteSettingsForm.
 */

namespace Drupal\module_name\Form;

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

/**
 * Configure site settings for this site.
 */
class SiteSettingsForm extends ConfigFormBase {
  /**
   * {@inheritdoc}
   */
  public function getFormId() {
    return 'site_settings';
  }

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

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

    // Build form object example:
    $form['test_1'] = array(
      '#type' => 'textfield',
      '#title' => $this->t('Title'),
      '#default_value' => $config->get('test_1')
    );
   
    return parent::buildForm($form, $form_state);
  }

  /**
   * {@inheritdoc}
   */
  public function submitForm(array &$form, FormStateInterface $form_state) {

    $this->config('site.settings')
      ->set('test_1', $form_state->getValue('test_1'))
      ->save();

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

}