ConfigFileStorage::initializeStorage in core/includes/config.inc

Create a configuration directory, if it does not already exist, and ensure it is writable by the site. Additionally, protect it with a .htaccess file.

file_ensure_htaccess in core/includes/file.inc

Creates a .htaccess file in each files directory if it is missing.

file_requirements in core/modules/file/file.install

Implements hook_requirements().

system_check_directory in core/modules/system/system.module

Checks the existence of the directory specified in $form_element.

system_requirements in core/modules/system/system.install

Implements hook_requirements().

telemetry_telemetry_data in core/modules/telemetry/telemetry.telemetry.inc

Implements hook_telemetry_data().