When you add a MySQL or MariaDB service to your project without any customization, you will get a single database called main, and a single user to access it also called main.
To configure multiple databases with more granular permissions, you can add a configuration key to the database service definition and configure multiple databases and optionally one or more custom users with more specifically defined access permissions.
For full instructions on setting up multiple databases, including example configurations, refer to the Multiple databases set up.
Note that when using multiple databases, passwords are automatically generated.