How to use functional key layouts?

[novoid]

In #1195 Samscryption pointed me to "functional key layouts".

In my HeliBoard 2.2 settings, I found "Advanced ..." -> "Customize functional key layouts": selection one of:

1. Functional keys
2. Functional keys (Symbols)
3. Functional keys (More symbols)

Where "Functional keys" looks like

So I guess I've found some default settings for functional key configuration I could already use and extend.

However, I was not able to find out how to actually use it. I presume I need to long-press some key and get "shift", "delete", "symbol_alpha" and so forth.

(1) Is this assumption true so far?
(2) Where do I need to long-press in order to see the functional keys configured above?
(3) What are the differences between "Functional keys", "Functional keys (Symbols)" and "Functional keys (More symbols)"?
(4) What is the process of adding my own keys to specific existing keys for long-press use?

I've read https://github.com/Helium314/HeliBoard/wiki/Layouts#what-a-json-file-looks-like-for-heliboard but it only explains syntax.

I've read #947 but I did not understand it. E.g., when I follow #947 (reply in thread) and copy my QWERTY to "QWERTY2", I end up with a file with q, w, e, ... each one as a character per line. It doesn't look like the syntax from the layouts wiki page at all.

So far, it's very confusing to me and I could not find any documentation on that.

Thanks for your help!