As per the Home Assistant documentation, the configuration file is located at:
/config/configuration.yaml
Configuration.yaml - Home Assistant
The backup folder is located directly on the root of the drive at /backup/
To access these files, I just installed and configured the SSH & Web Terminal Add-On, and used WinSCP to connect to my Home Assistant instance:
Home Assistant Add-on: Terminal & SSH
Home Assistant Community Add-on: SSH & Web Terminal - Home Assistant OS - Home Assistant Community
I hope this helps.