Every character is capable of at least some rudimentary crafting, provided that they have the necessary materials to do so. Crafting includes synthesizing new items from scrap, or the modification of existing items. [Depending on the difference in level or attribute requirements of the item being crafted to the character doing the crafting, there is a chance of failure.]
This differs from the synthesis of items from a character's Matrix, since it is actually the assembly of tools from physical items.
For making new items, the character uses the crafting menu.
The crafting menu can be filtered by items that the character has the recipes or blueprints for, the items the character currently has the components for, etc.
Repairing Items
Existing items can be repaired from their own display panel. Repair is typically a scrap metal cost, but more sophisticated items may require specific parts for repair (similar to like-weapons used for repair in Fallout 3). You can sometimes craft these special items yourself if you have the recipe or blueprint.
Examples of components for Modern items are lag bolts and microchips. Examples of components for Primitive weapons are gems and runes. An item can always be repaired by dismantling an item of the same type. For example, one shotgun can be used to repair another shotgun. The item being repairs recovered durability equal to have of the remaining durability of the consumed item.
Repairing broken items has a chance for failure (either leaving the item broken, or rendering it useless). Only mechanics can repair useless items.
Generally the repair cost for items is a small fraction of the cost for the unbroken item.
Modifying Items
The modification of existing items typically consists of adding or removing components, which behave much like sockets in PoE. Adding or removing components on Modern items typically carries a scrap metal cost.