I have previously talked about configuring a Drupal site to authenticate against a remote SimpleSAMLphp install, but as Drupal is an excellent user management system I wanted to turn it around and use Drupal as the identity provider. This means that Drupal would allow users to log into other systems using their Drupal username and password by leveraging the power of SimpleSAMLphp.
This can be accomplished by wrapping the Drupal site and SimpleSAMLphp together along with a couple of modules to power the communication between the two systems.
Read more here: https://www.hashbangcode.com/article/drupal-9-configuring-drupal-be-identity-provider-simplesamlphp