In Drupal 8, there are many ways to interact with configuration from PHP. Most will allow you to write the config values, but some are read-only. Some will include 'overrides' from settings.php, or translations. There are many layers of abstraction, that each have different purposes, so it can be easy to run into subtle unintended problems (or more obvious errors!) if you pick a method that doesn't quite suit what you actually need to do. Here I'm going to outline some of them and when you might pick each one.