Food for chicks is a moving target. The first two weeks, they are so small and so fragile, anything other than chick starter crumbles would be (in my opinion) putting their health at risk. Chicks that just forage in nature behind their mama hen have a very high attrition rate. You'll see mama hen lose over 50% of her chicks in the first two weeks that way. Getting them off to a good start is critically important.
By the third week, they want to start pecking at greens but they will not get enough nutrition that way. Their growing bodies need protean if you want them to start laying at 6 months. They are not big enough to eat worms or really know how to attack even a medium sized bug. In the wild, mama hen would peck at these bugs and help them tear it into small enough pieces for them to digest. But even something as small as a meal worm is too large for 3 week old chicks. Putting them outside, even for the day, may put them at risk. They still need to stay warm. If they get wet or chilled at this young age, they can easily get sick. They will start to scratch around and will want to give themselves a dust bath while outside, but they don't have the capacity to fully feed themselves. They'll peck at small grains but anything larger than an okra seed is probably too large for them.
By the fifth week, some of your larger chicks will figure out how to eat a worm (you start on one end and just keep gulping it down). But still, its doubtful that even with an environment where there is rich forage, they'll be able to supply their need for protean. They'll love a clump of lettuce or comfrey. If you mix their starter crumbles with a smashed-up banana or some left-over pasta, they'll gobble that down. By the fifth week, I'm moving my birds from the brooder to a day-cage outside so they can start to learn how to hunt and forage. I'll toss worms and grubs into the cage for them to peck at. Some will show interest. Often they'll just grab onto the grub and run with it, causing all the other birds to chase them. But actually swallowing larger bugs is still difficult. The majority of their nutrition is still coming from high protean chicken starter mix.
Only by week 7 or 8 do I feel comfortable moving them full-time from the brooder to the chicken tractor/coop. This is dependent upon the weather in your area. If it gets below 55 degrees or so at night, I'd still have a heat lamp available for them to huddle under. By this point I've usually begun taking them off starter crumbles and mixing in a greater ratio of grower mix. Grower doesn't have the high calcium that layer mix does. Too much calcium is really bad for them and can damage their growing kidneys and even kill them. If there are adult birds to show them the ropes, you can start let them free range. I do this by letting them out of the chicken tractor about an hour or two before sundown. They will not wander off too far. Once the sun sets, they'll move back to the coop (my coop is mounted on top of the chicken tractor -- up the little ramp they go for the night). By week 7 they are aggressive enough to chase flying bugs. They can scratch well enough to hunt soil bugs. They'll prefer natural foods to chicken crumbles, but I still make sure they have plenty to eat. They are still growing rapidly and you don't want them going bed at night still hungry, or you will delay egg production. A good grain for them at this point is milo or oats. Unless corn is crushed, it's still too large for them.
Integrating new birds with the old hens comes with its challenges. The old girls can be pretty hard on the new ones. The young birds will need plenty of space to get away if they start getting pecked. I will tractor them separately, and will let them out at night to forage as one flock, before putting them into separate coops for the night. I keep the rooster away from the young birds -- they don't need his affections just yet. Old birds are on calcium-rich layer feed, which isn't good for young birds. Even after week 10, they should still be eating grower, not layer feed. As for foraging, they are getting the hang of things. If its summer, the number of bugs available for them has jumped up considerably. You'll notice that they don't eat nearly as much food. They'll be able to take a full-grown comfrey plant down to the soil level in an afternoon. After week 10, they can handle most small grains and seeds. They'll attack a sunflower head and clean it up. Almost any grain except large dent-corn kernels are quickly eaten. They'll eat large grasshoppers and will easily eat worms and grubs.
By week 15, they are now one flock—no need for two tractors or two different coops. The young girls can transition to layer mix—I begin to wean them off grower at this point and just finish whatever bag of grower that I have before only giving them layer. They are strong enough to fend for themselves if the old girls start picking on them. They are actually quicker than the old girls so they'll out compete with them for bugs. They'll gobble down black soldier fly maggots like they are candy. Some of them are smart enough to know how to bend over a stalk of grain or grass to get to the seed-head. They can aggressively dig through compost by this point. They should have enough street-smarts/jungle smarts to keep an eye out for predators by this point.
I don't lose many birds. My survival rate to adulthood is at least 95%, so for me, the extra care it takes to slowly transition them to a foraging diet is worth it. Most are large enough to begin laying at 6 months, and I get full production by their first fall. I run Bar Rocks and Rhode Island Reds -- good foragers but not as aggressive in that regard as other breeds. I've heard that Buckeyes and Minorca do well as foragers, particularly for hot-weather climates. If you plan of raising a flock of birds that will get the majority of their nutrition from foraging, you might do well to get a Buckeye or Minorca rooster so that you can breed some of that bloodline into your flock.
Best of luck.