Well, transformations from one n8n item into multiple different items are always tricky @Anthony_CAVALIER. This is because most n8n nodes work their logic on each item.
As for learning I think asking here on the forum is a good starting point, don’t hold back with that 
In the example I’ve used vanilla JavaScript and shared my personal approach to learning it here, but this might not be the right path for you. Such transformations could, for example, also be done using JMESPath.