Object-oriented PHP utilizes classes and objects to organize code into reusable chunks. This approach helps us organize complex applications, such as Drupal, into modular code called classes that can be reused across the entire system.

Symfony is a set of reusable PHP components, and a framework for building PHP applications. Drupal makes use of various Symfony components.

Plugins are one of the ways that module developers can write code that extends Drupal. The Drupal Plugin API allows a module to provide functionality in an extensible, object-oriented way.

Dependency injection is a design pattern commonly used in object-oriented software architectures in order to support Inversion of Control.

Events are one of the ways that module developers can alter or extend Drupal without modifying existing code.

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.