"Do they buy rice at the supermarket?"
Chinese members of my family always refer to 大米 when they talk about (uncooked) rice. It avoids confusion with 玉米 (sweetcorn) and 小米 (millet).
米饭 is used when speaking of cooked rice. 大米 is used to refer to the uncooked grains of rice, like what you might buy at the store.