Compare, for example, a painting. If I make a painting with a 5' by
3' hole in it, that is not derivative of Starry Night.
Even if I paint in complementary art such that if you put SN in there,
it looks nice, that's probably not derivative. But if I bolt the two
paintings together, and ship copies of the whole thing, then
a) I'm shipping Starry Night, so its copyright holder gets a say.
b) the bolts aren't a creative work
c) This clearly isn't "mere aggregation"
So whether or not it's a derivative work, in the sense of any nation's
copyright laws, doesn't really matter.