Need internal pdf structure detail for containers

After flattening the PDF, I see form Xobjects in the following structure:

Catalog -> Pages -> Kids -> 0 -> Resources -> XObject 

Similarly, I need to know the structure detail for highlighted containers in the attachment(flatten_pdf_internal_structure.png). Please let me know the structure for the same. I need this information to use it in my library.

Will appreciate any help on this. Thank you.