For example, there’s ‘apple juice.’ Does it mean juice relating to, constituting, or of apple? I mean, is there a contextual meaning without any words between two nouns? I think a literal noun doesn’t modify a noun.
An attributive noun (or noun adjunct) is simply a noun that is used like an adjective, usually because we haven’t bothered to create a distinct adjective form of that word.
In the case of “apple juice”, the noun “apple” is modifying the noun “juice”. We would assume the juice was made from apples or at least taste like it was.
The similar “orange juice” is more interesting because “orange” could be an adjective for the color of the juice or a noun specifying the origin or flavor of the juice. Since all three meanings are probably correct, we haven’t bothered creating different words to distinguish between them.