Everything we disassembled before, everything was like the previous versions of Drupal. But with the development, everything is different. Although visually Drupal looks the same as Drupal 7, inside Drupal consists of Symfony components, its components, as well as many libraries. Therefore, you have to deal with the new plugin system, deal with OOP in PHP.
To begin with, we will start with the pretty familiar things Drupal hooks, then we will figure out how Drupal works with pages (route), add a new page, consider how Drupal processes requests.
As new articles and videos are added, I will expand this introductory article.