Not sure where you are located but that may have something to do with the confusion. In the UK they have two different kinds of grit, soluble grit (oyster shell is one example) and insoluble grit (rocks used in the gizzard to grind food). In the States grit just means the insoluble kind, used for grinding food.
Since chickens don't have teeth, they use rocks in their gizzard to grind up tough or hard stuff. Those rocks are called grit or insoluble grit, depending on where you are. The insoluble grit you buy is granite but since yours forage they will pick up all the grit they need for grinding the food in their gizzard. What kinds of rocks they use will depend on what kind of rocks are in your soil. So you don't have to worry about providing them grit for them to grind their food. If you want to you can buy some and toss it on the ground for them, but you don't have to.
Oyster shell does not work to grind their food. It dissolves in their digestive juices. Just like you their digestive system has acid in it to help in digestion. Oyster shell does not last long in that acid. The oyster shell is a calcium supplement, used to provide extra calcium for their egg shells when they are laying.
Laying hens and pullets can get extra calcium from a lot of sources, especially when they forage. Some plants contain a fair amount of calcium. Hard-shelled bugs and some other creepy crawlies they find and eat while foraging can provide calcium. If your native rock is limestone they can get a lot of calcium from that.
If you feed them Layer they get a lot of calcium from that. It doesn't matter how much calcium is in one bite, what's important is how much total calcium they eat in a day, end even that is an average over several days. What this means is that if Layer is all they eat, they can get enough calcium from the Layer for the egg shells. But since yours forage for at least part of their food, Layer by itself may or may not provide enough. A lot of that depends on how much other than Layer they eat but also what other calcium they find when foraging.
Chickens seem to instinctively know whether they need the oyster shell or not. The ones that need it for egg shells seem to know to eat it, while the ones that don't need it don't eat enough to harm themselves. I think it is a good idea to offer oyster shell on the side in its own container or scattered on the ground and let them decide. If they are getting calcium from another source that oyster shell can last a long time. If not they will eat it and you will notice.
I almost always have immature chickens in my flock so I never feed Layer. I offer Starter or Grower (depending on chick's age) to them all with oyster shell on the side. Since I always have hens laying, that oyster shell is always out there regardless of the chicks' age. Mine also forage.
There is no rush for you to offer oyster shell. The earliest I've ever had a pullet lay is 16 weeks. That's really rare by the way, don't expect an egg that early. I'd personally put oyster shell out by then so it's ready, but it also works to wait until you see your first egg. You can switch to Layer when you see your first egg or you can continue to feed another feed. There are no set rules for any of this.
Good luck!
