Drupal 8 - Directory Structure

Drupal 8 and that follow are more objected oriented and more structural.

Here are few of state-of-the-art drupal structure to order all parties files and code.

first level which also known as the Base Level directories contain the following folders:

/core - All files required by Drupal's out-of-the-box usage (core)
/libraries - All third party external libraries leveraged by Drupal. This folder is not included by core, but used with many contributed modules.
/modules - The directory into which all modules go, divided into two sub folders:
- /custom - All modules created/customize for the current website
- /contrib - All modules created by the community which system check if new release exist from time to time
/profile - All contributed and custom installation profiles
/sites - All files which defining the site and its services (backwards compatibility with drupal 7)
- /default or /domain
  - /files - The storage of site-specific files. This includes files uploaded by users (such as images) and site configuration
  - /modules
  - /themes
/themes - All themes installed, this folder also include two main folder groups:
- /custom
- /contrib
/vendor - Backend external libraries that Drupal core depends on

In addition, the folder structure in the /core directory has changed as well.

/core/assets - Various external libraries used by core (includes jQuery, underscore, modernizer, etc.).
/core/includes - Base-level functionality that Drupal uses through other /core folders.
/core/lib - Drupal core classes.
/core/misc - Frontend code that Drupal core depends on.
/core/modules - Drupal's core modules.
/core/profiles - Drupal's core installation profiles. These are Minimal, Standard, Testing and Testing multilingual installation profiles.
/core/scripts - Various command line interface (CLI) scripts, mostly used by developers.
/core/tests - Drupal core tests.
/core/themes - Drupal core themes.

Multisite structure

on base level directories: