Re: Why psql connection assumes default database name as the username

​Nowhere in "pg_roles" does the name of a specific database appear - so no.

You defines roles.  You define databases.  You give roles CONNECT permissions on databases if you want them to be allowed to connect to said databases.  Of course, superusers can connect to anything.  That is, along as pg_hba.conf allows them to attempt a connection in the first place.

What I said above is wrong, though, if one leaves the default permissions of the implicit PUBLIC group in place.  By default all users are allowed to CONNECT to any database in the cluster - among other permissions.

So while creating a database of the same name of as a user "just works" it is working because of a default that can be changed.  The fact that the names match doesn't enter into it.