In Drupal’s core library there are a number of utility functions and classes that, as a module developer, you will find make your task easier or less tedious.

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

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

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.

A reliable backup will allow you to restore your site if something goes wrong.

Use the Configuration API to access configuration data stored by a module, provide default configuration, create settings forms, and other tasks such as overriding settings for a local environment.

Cascading Style Sheets (CSS) files are added as an asset library through a theme’s libraries configuration file. HTML attributes, including classes and IDs, are commonly added in template files with Twig.

The Render API consists of a standard format for describing data using structured arrays and a process for converting those arrays into the HTML a user sees when interacting with a Drupal site.