Symbolism, Imagery, Allegory
In short, there are not a lot of symbols in realist fiction, especially in character driven realist fiction told by a first person narrator. One of the assumptions of the realist novelist is that symbols don't just exist in the world; symbols are created by people and they only gain significance as more men and women come to see them as symbols. So Giovanni's room becomes a symbol because David perceives it as a symbol. America becomes a symbol because Giovanni says things to David indirectly by talking about his homeland.