‘Whose’ here is general but takes on a slightly different genitive meaning of something belonging to (or used by, related to, etc.) the referent. ‘I saw a cat,

‘Whose’ here is general but takes on a slightly different genitive meaning of something belonging to (or used by, related to, etc.) the referent. ‘I saw a cat, which toy had black fur’ → the cat is a toy, and the cat-toy has black fur; vs ‘I saw a cat, whose toy had black fur’ → the cat has (owns, is playing with, etc.) a toy, and the cat's toy has black fur.