The default User is created during installation of the server instance. But you can query for a list of all current Users using this query:
SELECT usename AS role_name,
CASE
WHEN usesuper AND usecreatedb THEN
CAST('superuser, create database' AS pg_catalog.text)
WHEN usesuper THEN
CAST('superuser' AS pg_catalog.text)
WHEN usecreatedb THEN
CAST('create database' AS pg_catalog.text)
ELSE
CAST('' AS pg_catalog.text)
END role_attributes
FROM pg_catalog.pg_user
ORDER BY role_name desc;
For more information please see this article.
You can change the Users and their passwords using ALTER USER
, as discussed here. You can also disable the login for the default User as stated here.