Audit trail tables in Oracle 12c
The link that you have provided is related to Oracle Application not the database. The solution being explained on that website is the way to audit changes from application side. 

In Oracle database, we don't have such tables to hold changes.


  Is audit trail tables still the way to go or is this built into unified audit?


No, in unified auditing there is only one view called UNIFIED_AUDIT_TRAIL to display all the data related to the audit.


  Can we enable it through some sql statements, and if so, is there a guide for that? Or do we need to enable it through a tool?


Again, the link that you have referenced is related to Oracle Application not the Oracle database and there is way to do it in it. Audit Installations Window.

Better approach, from database side, is using Oracle Flashback Data Archive. 

This article from Oracle Magazine explains why Flashback Data Archive is a better way as compared to manual auditing using audit tables and triggers to track changes. 

A Fresh Look at Auditing Row Changes