The code you are trying to use is very old.
Currently there are no such cards.
Go to custom:hui-element card GitHub repo for info.
Besides, I am not sure that a person who wrote this code did it correct. These cards (currently named hui-element) are used to add a stack where a stack cannot be added - and “paste a stack into Entities card” seems to be the only use-case. In other cases a standard stack could be sufficient.
Honestly I am new to this and I would like to understand how to do this correctly, because in the end I am cutting and pasting but I don’t really understand the concept, based on your experience, where do you recommend me to go to read information about this and learn?
(w/o code) Any standard card or entity row inside Picture elements card. The card only accepts so called “elements” & custom cards; to paste a standard card (or an entity row) - you need to wrap it into hui-element.
(w/o code) (advanced) Inside a template for a “custom:decluttering-card”. The template accepts a CARD - but assume you need to create a template for standard entity row (like a slider) - then wrap it into hui-element.