I searched but couldn't seem to find the right info. Basically, I have read "how to backup or locate your opus configuration simple".
How can I backup just the preferences/settings of dopus? Is this stored in a single file somewhere?
Settings > Backup & Restore will only backup Opus-related things, not anything else, so that's all you need.
That was a lightning-fast answer. But what exactly is "Opus-related things"? I assume that includes toolbars, hotkeys etc? I am looking to backup only the preferences.
Which Preferences? Preferences as a whole includes references to things like toolbars, so it is all connected. 
The advanced version of the FAQ you linked covers all the different config files and what they contain. You can backup individual files if you want to, but it usually makes sense to simply backup everything using the wizard. The backup files are not large.
Ah I see. I thought maybe there was a separate file only for the changes made in the preferences panel, but that is OK.
The reason why I was asking was because I wanted to save only some settings with a backup, not the export function.