Grass is nutritious; if you don't overstock and let them free range they won't eat it to destruction. Weeds are nutritious, and will grow without any help from you, if you can live with an un-immaculate garden. Insects and small animals that chickens find and eat when foraging in the undergrowth contain amino acids that plants lack. All the minerals that a chicken needs in the quantities that it needs can usually be obtained from those sources. Obviously, if the ground is frozen or baked half the year where you live, it won't be the case, but whatever wildlife grows there is nourished from local resources, so look around, especially for pheasants, grouse, partridge and that sort of thing, and if they can thrive, so can your birds. Also, if you give as feed whole grains plus peas, they make a complete protein (ie together they contain all the amino acids that a chicken needs).
Most people who use this website do not live in an environment suitable for chickens to gather or scavenge their entire food needs for themselves, and/or they would not be happy with the reduced number of eggs or increased mortality that such a system usually produces, and you do not say where you live or what your intentions are other than saving money on feed. But in other parts of the world that's exactly how many chickens are kept; see e.g.
https://doi.org/10.1016/j.psj.2022.102062
which is about the diversity of smallholder chicken farming in Tanzania, and the pros and cons of the different systems, from entirely self-supplying to intensive commercial. It will give you plenty of food for thought.