How to create your own custom settings form in Drupal

custom_config_form.info.yml

name: Custom Config Form
description: This module illustrates how to create your own config/settings form.
core_version_requirement: ^8 || ^9 || ^10
type: module
configure: custom_config_form.settings

custom_config_form.links.menu.yml

custom_config_form.settings:
  title: Custom Settings Page
  description: Custom settings page
  parent: system.admin_config_system
  route_name: custom_config_form.settings
  weight: 10

custom_config_form.routing.yml

custom_config_form.settings:
  path: '/admin/config/system/settings'
  defaults:
    _title: 'Custom Settings Page'
    _form: 'Drupal\custom_config_form\Form\SettingsForm'
  requirements:
    _permission: 'administer site configuration'

SettingsForm.php

<?php

namespace Drupal\custom_config_form\Form;

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

/**
 * Configure Custom Config Form settings for this site.
 */
class SettingsForm extends ConfigFormBase {

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

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

  /**
   * {@inheritdoc}
   */
  public function buildForm(array $form, FormStateInterface $form_state) {
    $form['example'] = [
      '#type' => 'textfield',
      '#title' => $this->t('Example'),
      '#default_value' => $this->config('custom_config_form.settings')->get('example'),
    ];
    return parent::buildForm($form, $form_state);
  }

  /**
   * {@inheritdoc}
   */
  public function validateForm(array &$form, FormStateInterface $form_state) {
    if ($form_state->getValue('example') != 'example') {
      $form_state->setErrorByName('example', $this->t('The value is not correct.'));
    }
    parent::validateForm($form, $form_state);
  }

  /**
   * {@inheritdoc}
   */
  public function submitForm(array &$form, FormStateInterface $form_state) {
    $this->config('custom_config_form.settings')
      ->set('example', $form_state->getValue('example'))
      ->save();
    parent::submitForm($form, $form_state);
  }

}