Module

Step 1. Install Tomcat And Solr Server (atlest 5.4.1)

Step 2. Install Solr Search API On your drupal

#Drush# drush en search_api_solr -y

Step 3. Copy solr core files

Drupal 8 Devel module has an integrated switch user block.  This function allow you to switch users without logout/login (for the selected roles) and without entering user name and password. To activate a this block.

Features is a drupal contrib module enables the capture and management of features in Drupal, by providing a UI and API for taking different site building components from modules with exportables and bundling them together in a single feature module.

Drupal 8 Hook System

Hooks allow modules to alter the drupal core behavior or another module. Hooks are discovered and called at specific times to alter or add to the base behavior or data (invoking the hook). Each hook has a name start with hook_ (Ex; hook_menu()).

This system allow you to invalidate drupal cache using a 'tag'

Exapples:

You can't change the body of this block fron the user interface. But you can do that using a small custom module using hook_block_view_alter().

Example: in the yourmodule.module

You can find any entity using Drupal 8 EntityQuery System.

Example:

1. in yourmodule.links.menu.yml
yourmodule.test:
  title: Test
  parent: system.admin_config_development
  route_name: yourmodule.test

Module Name = my_module
Content Type = my_new_content

Create content type

Put this two files into my_module/config/install/

This is a drupal 8 contrib module allow  you (developer) to find code examples.