Medicated feed is to help build up immunity against coccidiosis (an internal parasite) which can be present in your soil whether you have chickens or not.
Grit is used to grind up food (other than commercial feed) in the chicken's gizzard. If you feed treats before they go outside where they can pick up small bits of stone to use in their girds, you need to supply some.. All purpose sand will serve as grit quite well. Play sand will not.
Crushed oyster shell is for layers. Chicks should not get layer feed with the extra calcium in it - not until they start laying eggs!
I don't use layer feed at all, but feed grower/finisher feed to everybody over 8 weeks old, providing oyster shell free choice.
Hope that helps!