It’s common to see opercula on the shore but not in the concentrations shown in these photos. I saw these outside a seafood restaurant, discarded as the creature is extracted from its shell.
An operculum is attached to the body of the shellfish and is used to seal it inside the shell. Wikipedia suggests that the purpose is either as protection from predators or to prevent shellfish that live in the intertidal zone from drying out. They’re no match for the Haenyo, the women divers of Jeju.