Drupal::config to access Site default timezone
Inspect the system.date configuration

You can navigate to the /admin/config/development/configuration/single/export path and inspect the configuration types and values. The default timezone is of type simple configuration and system.date is the cofiguration name.

Inspect all the configuration values

Go to /admin/config/development/configuration/full/export, here you can export all the active configuration, you can download and extract the bundle and then inspect the individual files. If you have the drush command line tool, you could use the command drush config:export.

Official Documentation

You should read the official documentation to better understand the logic behind the Configuration API. These are some links:

Simple Configuration API | Drupal 8 guide on Drupal.org

Configuration API | Drupal 8 guide on Drupal.org

Configuration API overview | Drupal 8 guide on Drupal.org