Lets be clear on this. The Fn key is a key mostly only on MacBooks that acts as a hardware toggle on the function keys, and toggles them between function keys (F1, F2, etc) and hardware keys (volume up/down, brightness up/down, etc).
The Fn key is not a modifier. It is not like Shift or Control. The is no such thing as Fn-K - Fn-K is just K. The Fn key only operates on the function keys (and sometimes a few other keys like Page Up/Down). And it does not operate as a modifier, it changes the key.
You can toggle the Fn key’s behaviour in the System Preferences, Keyboard preferences. That just toggles the meaning of F1 and Fn-F1, which one is the function key and which one is the hardware key.