Like many other action RPG game that involves looting, Fallout 4 has a lot of junk items you'll come across, and some of them are more scarce or essential than others. Here's a list of components you have to keep an eye out for, as well as the items you can scrap to get them.
1. Circuitry
You'll be needing circuitry for a lot of power-related crafts, along with defenses, armor modifications, and other structures. They can be hard to come around, but they are easy to acquire once you know where to look and loot.
You can find it in: Telephone, Circuit Board, Typewriter, Destroyed Turrets, Hot Plate, Military-Grade Circuit Board, Enhanced Targeting Card, Merchants
2. Adhesive
This item is essential for armor and weapon upgrades, including power armor modifications. You will also need it for crafting duct tape. You can usually find the items you can scrap to get this item around the map. Just make sure to keep an eye out on shelves and containers.
You can find it in: Duct Tape, Military-Grade Duct Tape, Pack of Duct Tape, Wonderglue, Economy Wonderglue, Sealed Wonderglue, Vegetable Starch, Merchants
3. Fiber Optics
If you intend to create high-tier electronic weapon modifications and certain settlement defenses, you will need fiber optics for that. They are hard to come by and can be expensive to purchase through merchants, so you should look for items you can scrap for them.
You can find it in: Applicator, Biometric Scanner, Microscope, Prototype Biometric Scanner, Ear Examiner, High-Powered Microscope, Merchants
4. Oil
It's easy to use up all your oil if you're not careful. It's crucial for a lot of settlement structures and equipment such as generators, as well as weapon and armor modifications. You'll get some by scavenging in Sanctuary Hills, but that won't be nearly enough.
You can find it in: Oil Can, Oil Canister, Paint Can, Soap, Used Oil Can, Yellow Paint, Lantern, Mr. Handy Fuel, Cooking Oil, Flip Lighter, Blowtorch, Aluminum Oil Can, Aluminum Canister, Gas Canister, Gold Plated Flip Lighter, Industrial Size Shortening, Merchants
5. Nuclear Material
You can already guess why nuclear material is important in the game. You can use it to make weapon modifications and some explosives. The reason they're hard to find is because the items you can scrap to get them are scarce, and the enemies you can loot them from aren't that easy to kill.
You can find it in: Biometric Scanner, Prototype Biometric Scanner, Blast Radius Board Game, Distress Pulser, Mini Nuke Items, Radioactive Enemies (Glowing Enemies), Alarm Clock, Wakemaster Alarm Clock, Ichor Sack
6. Ballistic Fiber
One of the rarest items in the game, you will need them to craft armor as well as repair them. It can be used to increase the damage resistance of your armor, which goes without saying can go a long way when facing strong foes.
You can find it in: Military Ammo Bag, Military Grade Duct Tape, Merchants
7. Springs
Springs are important for all sorts of settlement items and defense mechanisms like turrets. They're not as hard to find as ballistic fibers, but it might still take you a while to collect a whole lot of them.
You can find it in: Typewriter, Gold Watch, Handcuffs, Cameras, Alarm Clock, Antique Pocket Watch, Battered Clipboard, Giddyyup Buttercup components, Kitchen Scale, Toaster, Flip Lighter
