What is the kind of structures with a thin layer of material spread into a curving shape?

The structures you're referring to are often called "shell structures." These are thin-walled, curved forms designed to efficiently distribute loads while minimizing material usage. Common examples include domes, shells, and curved roofs, which are used in architecture and engineering for their strength-to-weight ratio and aesthetic appeal. They can be found in various applications, from buildings to bridges and even in packaging designs.