Postgresql : How to login as a NOSUPERUSER and create a database? After logging in as Superuser "postgres" I created another user "p5" :

testdb=# CREATE USER p5 NOSUPERUSER CREATEDB LOGIN PASSWORD '123';
CREATE ROLE
testdb=# \du

p5        | Create DB     
postgres  | Superuser, Create role, Create DB, Replication, Bypass RLS


To login as postgres(superuser) I used to do :

revoltman@G41:~$ psql testdb postgres
Password for user postgres: 
psql (9.5.10)
Type "help" for help.
testdb=# 


How to login as user "p5" and create a database "db5" ? p5 already has CREATEDB rights ! Should I create "db5" first as a Superuser and then connect "p5" to "db5" ? If so then what is the use of p5 having CREATEDB rights ?
https://stackoverflow.com/questions/31819377/psql-login-fails-database-not-found?rq=1
I think I understood it. I should connect to an existing database to login as p5 user. Then I create another database "db5".
If postgresql could have allowed login and then allowed to create databases then it would have been an intuitive way.