I do not understand why Item keep a compact format.

Using expand function on an OData endpoint doesn't mean it will return a flat table.

It means that you are requesting related data, that are not by default provided in the response. Expand function can be used for complex types and collections.

How should I proceed to do so?

By the image, I guess, you are working in Power Query which offers JSON and XML parsing functionality. I would suggest following Parse text as JSON or XML (Power Query).