Hey @antonioreale good question! Each resource should have an example page/code box that goes over each of the fields available on its page in our developer documentation. For example, the product resource here in REST shows each available property on the resource. In GraphQL, nested object fields can be a little trickier for sure, but let's use the product object in GraphQL as an example here. We can look at the nested fields and connections on the right side of the page:
If you click into the page for each nested field on the product object, it will show you the fields and connections for each respective sub-field in the same sort of nested menu. Let us know if we can clarify anything further on our end - hope this helps!