Drupal forms are built using the Form API (FAPI). Sometimes you control the original form, but very often you need to change an existing form that belongs to core or a contributed module.

This is where form alters come in.

In the previous topic, we learned how routes map URLs to controllers. Now we move one step deeper into backend Drupal: what controllers return.

In Drupal, routes are the entry point to backend logic.

Every page, controller, form, AJAX callback, or JSON response starts with a route. If custom modules define what behavior exists, routes define how a request reaches that behavior.

A custom module is the primary way backend behavior is added to a Drupal site.

Custom Configuration Entities are a core architectural concept in Drupal that allow developers to model structured configuration as first class objects. They sit at the intersection of the Configuration Management Initiative (CMI) and the Entity API.