Some resources are common, others are uncommon, an some can be rare. All resources (and items) are shared between workbenches and crafting stations within a settlement, and can be shared between settlements by establishing a supply line using the Local Leaderperk. Unassigned settlers will automatically scavenge for the resources, and scavenge more when their level of happiness is elevated.
There exist unused component models for wood, rubber, concrete, and a generic metal. These resources were presumably the first implemented, and were eventually replaced with the currently existing system.
↑Fallout 4 loading screen hints: "Every Workbench, Crafting Station and Modding Station in a single settlement shares resources. Using one will grant you access to anything placed in the others."
↑Fallout 4 loading screen hints: "With the Local Leader perk, you can link two workshops with a supply line. They will then share food, water and any resources needed for building."