on top of what everyone else said, i'd add oats, bananas, whatever cheap but decent peanut butter you can find.
hit some sort of discount food chain that has house brand versions of more expensive stuff if you need something like almond or oatmilk.
don't buy white bread, because it wont fill you.
now i don't advocate stealing from local markets, but apples or bananas can be acquired for free with relative ease as long as you only pick one or two of each and forget to put them back.
if you want to flavor up your rice for cheap hit up an asian food store and buy some laoganma or some other spice paste or dip. they also have better and cheaper soy sauce and soy sauce can be a lifesaver when your pantry is otherwise dead and you want to have some flavor with your rice. good source for cheap rice or ramen ingredients.
learning to make a bunch of soups is good. soups are usually cheap to make, can be stretched out, frozen and reheated, and a few ingredients can go a long way. for some cheap carbs you add potatos, or rice on the side, or noodles. you can add beans to make the soup more filling or a boiled egg. i was living off of lentil soup all winter.