open a library I already migrated

I want to open a photo library that is already migrated. Elsewhere in Apple Support it says you can use Aperture 3.6 but I can't find it except from websites I've never heard of. How can I safely open a library that has already been migrated? Moving to Big Sur erased some of my photos. When I compare the library that is listed as already migrated and the current library in use, the former is larger, so maybe those missing photos are in there? I want to open it and check.