OpenCms does not create it's own database users. During the installation, OpenCms asks for two database user accounts. These users must have been created before starting the setup.
That administration user account is only used for creating a new database and all needed tables inside of that database. After Setup, OpenCms will not keep any information about this user account and will not use this user account again.
For production access, OpenCms needs a database user. OpenCms also asks for that second user account during that setup. As the Administration user, this user must have been created before setup. OpenCms will store the user name and password of this user in the configuration file opencms.properties. After setup OpenCms will continue to use this account for connecting to the database.
The database user account that is used for production should have reduced privileges. It should not be able to access other databases, nor should it be allowed to create or delete any databases or tables.