initdb just sets up the directory structure and such that is needed to create new databases. To create a database, use createdb:
SYNOPSIS
createdb [ option… ] [ dbname ] [ description ]DESCRIPTION
createdb creates a new PostgreSQL database.Normally, the database user who executes this command becomes the owner
of the new database. However a different owner can be specified via the -O option,
if the executing user has appropriate privileges.createdb is a wrapper around the SQL command CREATE DATABASE [create_database(7)].
There is no effective difference between creating databases via this utility and
via other methods for accessing the server.
initdb is sort of like creating a new file system on a hard disk: first you create the file system (initdb), then you create a files and directories (createdb).
The actual database files will be under /usr/local/var/postgres after you create the database. So, just create a database and then see what’s new or changed under /usr/local/var/postgres. There isn’t a single “dbname.db” file or anything like that, each database is a collection of files with names that are only meaningful to the database server.