HOLLIS has two main parts. There’s the traditional Library Catalog of Harvard's books, journals, films, sound recordings, maps, and so on. Then there’s a huge additional database of articles, reviews, etc. which come from various sources outside Harvard. Many of these are available online through Harvard's subscriptions.
You can search the Library Catalog separately, or -- especially if you want to see articles -- search for "Catalog and Articles" at once. These choices appear as soon as you start to type a search.
Research tools like HOLLIS and library databases are specifically designed and curated for academic research, with more comprehensive results than you might get from an AI tool (because much published scholarship is not available for AI to train on).
Search limits such as these appear on the right-hand side of the HOLLIS screen when you do a search. Most databases provide similar ways to refine a search.