The engine speaks IDML: frames, stories, paths, tables, swatches. Everything else a publication needs is a plugin written against that engine — and each plugin keeps its content in its own native format. A workbook stays an XLSX, a layered image stays layers, a data session stays its sources and queries. They travel together inside one .paged file, which is itself a valid IDML package, and what lands on the page is native page content InDesign can open.