Welcome! I am fairly new to all this myself, but this is what I learned and what I do with my chickens...
Scratch - I give more in the winter as it is said to help put on a little more fat and in turn, keep the chickens warmer in cold weather. Not everyone agrees with this, but either way, it's extra feed and a special treat. I just toss scratch out into their run and let them "scratch" for it. It does not take the place of feed. Think of it as a "side dish" or "dessert".
Oyster Shell - my chickens are not quite laying age yet, but I already throw back crushed eggshells from organic eggs that we eat. I will probably be supplementing with some oyster shell if I notice their shells need a little help after they start laying in a couple months.
Granite - I can only offer "supervised" free-ranging, so because they don't get out of their run all the time, I do provide granite in a little dish. They will eat what they need. Sometimes I shovel in some rocky earth that is outside their run so they can pick the type and size of gravel they want right out of the dirt.
Feed - I fed crumbles until I switched to all-organic feed. The organic feed that I have access to for the best price is a "mash" - the only way I can explain it is it seems like a bag of chopped up weeds with little grains and dried peas in it. I ferment the mash and feed it to both my chickens and ducks. It is a "flock raiser" or "grower" feed. I buy it from
www.azurestandard.com which is a co-op, and we happen to have a local drop here. I did the math and figured out it was the least expensive organic feed I could get in my area. It is only a couple dollars more than the Purina flock raiser crumbles. Too me, it was worth the couple bucks to go organic - it is also soy free.
I personally like the way this feed ferments much better than the crumbles, but that is just my opinion. Of course, you don't have to ferment it.
I also throw out veggies and leftovers whenever we have them.