Services are objects that encapsulate the code for performing specific tasks in a reusable and decoupled way.

Drupal's Routing API maps the URL of an incoming request to the code responsible for generating the content that is rendered in response.

Controllers are the portion of a Drupal module that handle responding to an incoming request with the appropriate response data.

Composer is the preferred dependency management solution for PHP, is used by Drupal core, and is becoming increasingly popular for managing the modules and themes used for a Drupal project.