Apple Intelligence
Ask across collections and your library
Asking about a collection
Open a collection's "⋯" menu and choose Ask to open a chat scoped to every document inside it, so "Compare these documents" only compares documents you've grouped together, never your whole library.
Asking about your whole library
Ask, from the library's toolbar, widens the scope to everything you've added. "What's in my library?" and "Find documents about…" both draw on your full collection.
Note
A question that needs more context than the on-device model can hold at once will come back asking you to narrow it: try naming the document or collection you mean instead of asking broadly.