I would look at some of the posts in these discussions:
https://forum.inaturalist.org/t/way-to-see-unobserved-species-for-project/38200
https://forum.inaturalist.org/t/a-query-that-will-find-unobserved-species-in-a-smaller-place-that-exists-inside-a-larger-place/44807
For example, this is the compare tool, showing the bird species seen in Ottawa, but not seen in the Petrie Island project.
