drupal-project directory structure 8

The directory structure of the popular drupal-composer/drupal-project uses "/web" as the Drupal project root. Clarification could be helpful for beginner/intermediate developers.

What is the current best-practice placement of everything (files/folders, etc) in a Drupal project in relation to web document root -- in a sub-folder (like the drupal-composer/drupal-project appears to use) -- or in web document root proper.