Personal use case: add fediverse symbol ⁂ character to f key.
You may need to add some mathematical symbol/character, language symbol, etc. that Heliboard's default layout and dictionary may lack of.
qwerty.txt.my-unicodes.txt.f ⁂ 🖕 , space separated in a single line.+ to add a new layout and in the pop-up touch Add personal layout
my-unicodes.txt file (in the picture I use «unicode-heliboard»)Finally you have to select the keyboard you have added the characters to (long press on space bar).

just long press on the key you added the character to and it'll be available as any the other options you already had in that key.

That's it. You then can directly add new characters editing this file.