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.

Themes provide the HTML, CSS, JavaScript, and other assets that are responsible for the look and feel of your site.

Twig is a template engine for PHP, which is the language used in Drupal. Twig can be used to design templates in generic PHP applications as well as Drupal themes.

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.