This tutorial is designed to address relatively advanced use cases when theme developers want to create links to content outside of the template's scope. This seldom occurs in templates. If you're editing the template for a specific entity type, example node.html.twig, there will almost always be a {{ url }} or similar variable that you can use to link to the entity in question. Make sure you check for an existing variable with a URL first.

Example from node.html.twig:

In this tutorial we will learn how to add items to the menu system from within a custom module. We will also look at the options available for configuring our new menu item. Lastly, we'll learn about using hook_menu_links_discovered_alter() that can be used to add new menu items, or alter the behavior of existing ones.

Each form is defined by a controller, a class that implements the \Drupal\Core\Form\FormInterface. Form controllers declare the unique ID of the form, the $form array that describes the content of the form, how to validate the form, and what to do with the data collected.

In this tutorial we'll:

The Drupal Entity API makes it easy to define new custom content entity types. And in most cases whenever you want to store custom content in Drupal you should use the Entity API instead of making CRUD queries directly to the database. This ensures your code is more consistent, and allows you to take advantage of all the features of the Entity API like access control, Views module integration, and automatic JSON:API support.