In American and Canadian English, a "hamburger" is usually served in a bun, but can be served by itself, in which case it is commonly covered with gravy. Without the bun it is formally called a "Salisbury steak", but sometimes (perhaps this is a Canadianism) a "hot hamburger". (Yes, I know these idioms make no sense.) The word "hamburg" is not usually used in a culinary sense. When the patty is raw it is usually called a "hamburger patty", but often raw ground beef is sold as "hamburger" (no indefinite article and no plural).
I don't know what the subtleties of British usage are.
In Japan (or in Japanese), a "hamburger" refers to the dish with a bun, and a "hamburg" to the meat patty, whether it is in a hamburger or is served by itself with a vegetable side dish.
I'm not sure where else this distinction between "hamburger" and "hamburg" may exist. How are these words used in Malaysia?
Student or Learner