This question should be posted in the Multitenant forum
I've recently installed a virtual machine with a preconfigured 12c xe database.
That link installs an enterprise version of the database, NOT an xe version.
I created a database through the dbca tool and managed to create tables and import data.
Why? You just said you already have a database preconfigured based on that template so why are you creating another one?
Why aren't you just creating a PDB in that existing database?
I get the idea that I need a common user in a common database space (in root) that starts with a c##.
No - you only do that if you NEED a common user. You don't necessarily need ANY common users other than those created by Oracle.
I suggest you read, or reread, both chapters 17 and 18 of that concepts doc you linked to. Those chapters discuss common and local users and how to create them.
Here is what I get when I want to connect there :
sqlplus / as sysdba
SQL*Plus: Release 18.104.22.168.0 Production on Tue Jun 10 15:37:09 2014
Copyright (c) 1982, 2013, Oracle. All rights reserved.
ORA-01017: invalid username/password; logon denied
You 'get that' when you try to connect WHERE? To the CDB? To one of the PDBs? What are you trying to connect to?
What instance is running on the machine? What PDBs exist on the machine? Have you created a LOGON trigger to OPEN those PDBs when you start the database? Otherwise the PDBs will NOT be open.