AFAIK there isn´t such a thing.
You are free to create users/roles/groups in Postgres without a match in the Linux system.
They are completely separate things, and you would only end up with the same OS users and DB users if you intended to do so by naming them equally, but still they are different kinds of objects, like cars and fruits.
For convenience and security, the POSTGRES OS user is usually created in the installation process and is related to the POSTGRES DB user by the configuration set in PG_HBA.CONF
