米 as rice specifically means uncooked rice. Cooked rice is 米饭 (mǐfàn) or just 饭. 大米 (dàmǐ) means "rice" as a grain/commodity.
culture
Rice is the staple food of southern China, while northern China traditionally relies more on wheat (面食 — noodles, steamed buns, dumplings). The divide roughly follows the Qinling Mountains and Huai River.
