We could add moredetail to meal planning. You can save the most money by mastering "piggybacking," or rolling leftovers onto a new meal.
@baymule shared an example in an earlier comment!
Experienced frugal cooks often start with a bone-in roast (or a rotisserie chicken, which might cost even less!).
Eat the choice meat the first day, and pick the rest of the meat off the bone. Save the picked meat. You can usually get 2 meals out of it. You can mix with rice or another grain to stretch it for tacos or sloppy joes. (Save the bones and all your veggie scraps, like onion peels)
Put the bones in a big pot of water and boil them all day or night. (Crockpot or instant pot if you're not home to supervise). Ladle off the broth. You can make soup, keep it in the fridge up to 5 days, add salt and drink it, freeze it, or pressure can it. Refill your pot with water and repeat. You can keep making broth from bones until you can smash them between your fingers. If you can't use or preserve that much broth, freeze the bones and make more broth later.
You can also piggyback in less obvious ways. I cook chili 3 to 5 days after spaghetti. Why? I can toss any leftover marinara into the chili. Adds bulk and you'll never taste the difference. Nothing wasted! Then in a few days when my family is tired of chili, I make mac n cheese and mix it in with the chili for an easy goulash.
Little bits of leftovers can be reincarnated as quesadillas or grilled cheese.
Leftover oatmeal can hide in sloppy joes or meatloaf (if its not too sweet), or in bread, muffins, or cookies.
So many ways to piggyback!