Multi-site: Same Code, Different Databases
As easy as it is to just duplicate an existing codebase, it's not necessarily a good idea to have dozens of identical website folders if most of this codebase could be shared.
Did you know that most Drupal sites using the same version, share around 99.8% of the source code? Drupal core, libraries, modules, themes... everything comes from the same place: Drupal.org. And for organizations that share the same functionality and look and feel across multiple sites, this number is even higher.
Drupal introduced multi-site support in 2005 (yes, 12 years ago) so that multiple websites could share the same codebase, but with different databases.
How to Create a Drupal Multi-site
Just like with previous versions, you can create a new installation either from scratch or as a sub-site of an existing Drupal installation. The only thing you need is a new empty database.
For a detailed list of steps, you can take a look at Drupal.org Docs about Multi-site - Sharing the same code base.
Using drush, you can run this simple command: