Schematics act as a recipe for crafting items which detail the required materials (which may be components, resources and/or junk) and/or base items. Crafting an item is impossible without the appropriate schematic. Basic schematics can be obtained by constructing & upgrading the Weapons Bench and Gear Bench for weapon and gear schematics, respectively, while additional and typically rarer schematics can be scavenged during missions.
Schematics obtained from upgrading benches are available immediately, while those obtained through scavenging must be unlocked in the inventory, which will then permanently unlock the item for crafting. Scavenged schematics can be obtained multiple times and each one, even if they're for exactly the same item, takes up a separate slot in the inventory. Once a schematic is unlocked, any further copies of that schematic cannot be unlocked and can only be disposed of or traded.