system.rss.yml

aggregator_page_rss in drupal/core/modules/aggregator/aggregator.pages.inc

Page callback: Generates an RSS 0.92 feed of aggregator items or categories.

node_feed in drupal/core/modules/node/node.module

Page callback: Generates and prints an RSS feed.

RssFeedsForm::buildForm in drupal/core/modules/system/lib/Drupal/system/Form/RssFeedsForm.php

Implements \Drupal\Core\Form\FormInterface::buildForm().

system_update_8010 in drupal/core/modules/system/system.install

Move RSS system settings from variable to config.

channel:
  description: ''
items:
  limit: '10'
  view_mode: rss
langcode: en