Configuration of multisite on Drupal 8 is similar to previous Drupal versions. However, unlike previous versions, Drupal 8 requires multisite to be explicitly enabled before additional sites are recognized.
Enabling multisite with sites.php
Multisite requires the existence of the sites.php file in the sites directory. This file does not exist by default. If this file does not yet exist, copy sites/example.sites.php as follows:
$ cp sites/example.sites.php sites/sites.php
Each site in a multisite installation is signified by a directory under the sites directory. Drupal is installed with a sites/default directory. Additional sites are represented by creating additional directories. For example:
Each new site directory must have a settings.php file in order for Drupal to recognize it. On initial installation of a site, copy sites/default/default.settings.php as follows:
$ cp sites/default/default.settings.php sites/[new site directory]/settings.php