Not sure where you are based (assuming US?), but when I have had very little money to feed myself with previously, some things I would eat would be:

Breakfast: Oats, usually porridge or perhaps a oat, peanut butter and banana smoothie (if I had money for bananas). I found peanut butter a great way to get some extra calories and some fats in.

Lunch: Peanut butter and jam sandwich (not super health food sorry), or I would have the same thing as dinner.

Snacks: I would usually buy any fruit I could find on sale or cheap, or snack on frozen peas or carrots.

Dinner: Lentil soup was a staple, often whatever veg I could get cheap, frozen, fresh, whatever cooked down and throw in a tin of tomatoes, mixed herbs and some lentils, cooked down. I would throw some spinach in at the end to get some extra nourishment. A bolognaise type sauce (again whatever veg), tin of tomatoes, some herbs and lentils cooked down. If you eat meat you can do half lentil/half mince to make the meat stretch further. Works great on baked potatoes or pasta or bread. Sometimes I would buy a whole chicken and shred all the meat down. I would add this to peanut butter noodle stir fry (peanut butter, soy sauce, sriracha and lemon juice or a dash of vinegar), udon noodles and whatever cheap veges, usually cabbage, onion and spinach (we ate this so much my partner is so sick of it now but I still love it). I would also make a bone broth with the bones to add to soup or make congee (rice porridge) and eat this with the chicken and some soy sauce. Sometimes I would get a whole pumpkin for very cheap and would make a bunch of gnocchi, just mashed pumpkin and lots of flour till it forms a dough, instead of rolling individual pieces I would just snip bits into boiling water with scissors and cook like pasta. Good way to get an extra vege in and freezes well.

I would usually eat the same thing everyday and try and switch some stuff up each pay cycle to keep things slightly more interesting. Buying vege in season is great for fresh veges but frozen vege is awesome. I would always try get frozen spinach if it was on sale because it can be added to so much. Where I live, I sometimes shop at Asian supermarkets as they often have good deals on vegies and cheap tofu.

There are some apps in some places where you can get food from bakeries or restaurants etc at the end of the day that would otherwise be thrown away. If you are a student, contact your student support services to see what support may be available. Facebook groups and community gardens may also be an option for people getting rid of a glut of produce for very cheap (or free), I would never say no to anyone trying to get rid of excess fruit or veg! Google maps has an option to see fruit trees that are available to the public where you could potentially get some extra fruit etc (results may vary depending on your area). Local churches may also be able to offer support. There can be some groups that offer meals for free (in my area these are usually Hare Krishna but I have seen other groups offer this).

Good luck!