Really depends on the situation.

If I'm just feeding myself, I have no issue with going outside and foraging for food. I don't hunt, but I'm not the type that needs an animal based protein main entree in my meals, so it works/worked for me to collect wild vegetables, fruits, and fungi.

And from there, I eat whatever is cheapest. Grocery store mark-downs and deep-discount sales would guide my decisions. If an acquaintance was giving away food, I'd take it. When the food bank is doing a giveaway and it was close enough for me to visit, I'd go there and take what they had to offer.

At my poorest, when I had no access to a kitchen, peanut butter sandwiches were a mainstay. Tuna sandwiches were next best, but more expensive. At the time, powdered milk was a bit of a luxury, but it definitely helped wash down the peanut butter and was way cheaper by volume than fresh milk.

A lot of stores and restaurants, at least where I live, will have condiment packages out in the open. Don't go hog wild, but my experience is nobody cares/notices if you grab a few packs of whatever items are out: ketchup, mustard, mayo, honey, hot sauce, soy sauce, salt, and pepper -- in moderation -- so those can be free to you to use for meal prep.

When I've just been broke and/or saving money, my main protein was usually chicken. I'd just buy whatever was cheapest on sale, and try to stock up a bit or get rain checks. Then I could cook that in a crock pot and literally have meals for days. Around Thanksgiving and Christmas, turkey usually goes on deep discount and there are almost always a myriad of programs that just give them away. If you have room in your freezer and a crock pot, then you can be set just from that.

Add in some rice and/or beans/legumes to soak up the flavor when cooking meats.

Eggs were also always a solid choice, pretty versatile because they could be hard boiled, scrambled, fried, mixed into other things like noodles, or used to cook/bake other dishes.

Potatoes were another cheap source of carbohydrates, something that goes on sale often enough that I could usually find a deal, and if properly stored (cool, dark, dry) they can last a long time. Plus, they can go into the slow cooker with some chicken thighs and both ingredients benefit flavor-wise.

So, meals would be whatever combination of those things you can physically obtain. Your meal items don't have to have a name. If you have potatoes and mix those with scrambled eggs and mix in some wild dandelions, that's still a meal even if that's not going to show up in a recipe book. If you boil some noodles and add in some mayo and a pinch of rosemary from a bush you saw down the road, that's still a meal. Basically, just get creative with what you've got.