Every name brand is going to have its proponents and detractors. Some people swear by Dumor, some swear at it. Same with the others.
If they are of laying age, they need a feed that is about 16% protein and has somewhere over 4% calcium. If that is all they get, then they will do fine. It is called Layer. It can be this simple if you allow it to be.
Which brand? That depends on the last person you talk to. We all have different criteria and preferences.
Where it can get complicated is that we usually don't just feed them Layer. We give them treats or supplements. Some people add things to drinking water. This is usually not a big deal because chickens have a wide tolerance of conditions where they can do well. If they free range, they don't wake up and say "I need 12 June bugs, 21 earth worms, 31 Japanese beetles, and 2 wasps to meet today's requirements. Oops, that was 13 June bugs. What am I going to do!" They pretty much balance it with green stuff, seeds, and creepy crawlies, whatever they can find. As long as they are within general tolerances, they are fine.
If we feed them a lot of stuff that is low in protein, then the 16% protein layer may not give them enough protein. Some people that do that feed a higher protein ration to balance it back out. It is not about what percent protein they eat, it is about how much total volume of protein they eat.
Scratch can be many different things. It is often corn, whether cracked or whole does not matter. Scratch usually is lower in protein but higher in fat than their regular feed, and they will usually pig out on scratch and ignore their regular feed. Think of a child with candy. It does not have as much nutritional value as their recommended diet, but most children sure prefer candy. I like it too. If your chickens are confined and basically all they eat is what you give them, it is generally recommended to not give them more treats than they can clean up in 15 to 20 minutes and their diet will remain balanced. Many of us violate that rule, by the way.
Oyster shell is to provide calcium, mainly for the egg shells. Laying hens need a lot of calcium. If all they eat is Layer, they should get enough calcium from that. If you are feeding a lot of other stuff, then they may not. Or they may get even more calcium form hard shelled bugs, gravel they use as grit if you are in limestone country, or from some plants. If your egg shells are hard, they are getting enough calcium. If the egg shells are not hard, then you need to provide more and oyster shell is a good way to do that. I hang a container on the side of the run and put oyster shell in that. That way, if they want it, they can get it. If they don't want it, they can ignore it. Mine mostly ignore it, but I keep it available any way. A small bag lasts me a very long time.
You can make it as complicated as you want or as simple as you want. Your choice. As long as you stay within some pretty wide boundaries your chickens will do OK.
