I usually just pick the Key or combination of keys that I want to use. If that shortcut is already in use as another command - KDE will give you an window popup error message and you can decide then if you want to change it or not.

A lot of the keyboard shortcuts are found by going to Control Center -> Regional & Accessibility -> Keyboard shortcuts. There is a long list there.

Also there are shortcuts that can be assigned within most of the KDE apps you would check these individually within each apps settings.

Additionally there is also some shortcuts listed in the Input Actions section of Control Center.

I personally use Shift + Home key (N810) to trigger "Move Window". Not sure how using the Fn key would work because KDE might just think you're wanting to type a number/symbol.

Hope that helps