How do I tell Drush where the root directory is? I have successfully installed Drupal 8 with Composer and I also installed Drush with Composer. When I use Drush, it says that the Drupal root is not found. I am passing the root, but it still says that. 

How do I tell Drush where the root directory is?

I am using Drush from vendor/drush/drush/drush. I couldn't get vendor/bin/drush to work.
When I run:

$ composer create-project drupal-composer/drupal-project:8.x-dev drupal-project-test-site --no-interaction
$ cd drupal-project-test-site
$ vendor/bin/drush status


Then I see:

 Drupal version : 8.6.13                                                                  
 Site URI       : http://default                                                          
 PHP binary     : /usr/local/Cellar/php/7.3.0/bin/php                                     
 PHP config     : /usr/local/etc/php/7.3/php.ini                                          
 PHP OS         : Darwin                                                                  
 Drush script   : .../drupal-project-test/drupal-project-test-site/vendo 
                  r/drush/drush/drush                                                     
 Drush version  : 9.6.0                                                                   
 Drush temp     : /tmp                                                                    
 Drush configs  : .../.drush/drush.yml                                       
                  .../drupal-project-test/drupal-project-test-site/vendo 
                  r/drush/drush/drush.yml                                                 
                  .../drupal-project-test/drupal-project-test-site/drush 
                  /drush.yml                                                              
 Drupal root    : .../drupal-project-test/drupal-project-test-site/web   
 Site path      : sites/default                                                           


Drush should be able to find the Drupal root for the site that it's contained inside without the use of --root or an alias. I'm not sure why vendor/bin/drush would not work. Perhaps try again with a new test project, and see whether you get the same results? Perhaps your install did not complete.