It's uh... yeah. It's a little complicated. And unfortunately the game doesn't make this at all clear.
It all revolves around the Workshop.
Quick summary
You can break down five types of items into their components:
- Moddable weapons: you must manually break these down with the Weapons Workbench.
- Moddable armor: you must manually break these down with the Armor Workbench.
- Non-moddable armor and clothing: you must break these down by dropping them on the ground while in the Workshop menu.
- Junk: these will be broken down automatically for crafting; however, if you want to manually "liberate" specific components for any reason (e.g. quests or easier transportation) you can break down junk by dropping it on the ground as above.
- Neighborhood debris: you must break these down by wandering around while in the Workshop menu.
Three benches affect component creation:
- Weapons Workbench
- Armor Workbench
- Workshop bench. Though this looks like simple a workbench like the others, it's the heart of the crafting system and has an inventory that's available for the other benches.
When broken down, the components go to different inventories:
- To your character's inventory: components broken down at a workbench.
- To the Workshop's inventory: components broken down through the Workshop menu when you've dropped them or find them out in the neighborhood.
The components that are available to you for crafting are a combination of:
- Actual base components (like Plastic) and potential components from junk items (like the plastic in a Plastic Plate)
- Materials in your character's inventory and materials in the Workshop's inventory
To get needed components, the Workshop:
- Will automatically break down junk items, whether in its inventory or your character's inventory
- Will not automatically break down any other items (armor, weapons, etc.)
Breaking down junk inventory
You don't have to break these down to access their components.
Let's say you have a Plastic Plate in your inventory. When broken down, that's worth two plastic. But those two plastic are available as raw materials without a separate step of breaking them down.
These materials are available for Workshop and Workbench crafting whether they're in your inventory or the Workshop's inventory.
It's also possible to break them down manually the same way as for neighborhood debris as described later: by dropping them on the ground and manually scrapping each one. (Infrequently when you drop an item on the ground it will disappear—falling through the ground or into another object—so you might want to back up before doing this.)
Why would you bother to break down junk manually? If you plan to keep these items in your inventory for their components, for example to move them to another settlement, the components themselves will usually be much lighter than the original item. Note the usually though. Examples:
- 10lb Lead Weight (10lb) = 3 Lead (0.9lb)
- Extinguisher (6lb) = 2 Asbestos, 2 Rubber, 4 Steel (1.2lb)
- Bag of Cement (8lb) = 5 Concrete (15lb)
(That last one is presumably because concrete = cement + sand/gravel/stone.)
Breaking down moddable weapons
Moddable weapons act differently from junk in that you must break them down at the Weapons Workbench.
Let's say you have an Automated Institute Pistol, which includes plastic as a component. That potential plastic is not available for crafting.
To make it available you must go to the Weapons Workbench, select the weapon, and use the Scrap command. This will place those components in your personal inventory.
While this is an extra step, it makes sense. If the Workshop automatically melts down a wrench in your inventory, that's great. If it melts down your beloved laser rifle... yeah. That sucks.
It is not possible to break down non-moddable weapons (for example grenades), even by dropping them and using the Workshop menu.
Breaking down moddable armor
Armor that can be modded works the same way as weapons: you must break it down to make its components available. You break it down the same way, but using the Armor Workbench. This will place the components in your personal inventory.
Breaking down non-moddable armor and clothing
Other apparel, including clothing and non-moddable armor like helmets, must be broken down the same way as neighborhood debris, which is described in detail next.
As with moddable armor and weapons, the Workshop will not break down these materials automatically. You must do it manually.
Breaking down neighborhood debris
For all the debris near a Workshop—walk up to the Workshop bench, choose the Workshop option, and then walk away from the Workshop.
As you walk around your neighborhood, items will get highlighted with the option to Scrap them. Choose that option and they'll get broken down -- you can even break a car, tree, or collapsed house into its components in seconds.
Unlike the Weapons and Armor benches, breaking items down this way places them directly in the Workshop inventory rather than in your own. This makes sense: if you break down an entire house, you don't want to carry it around in your pockets.
While in this mode, you can see the Workshop's area of influence as a translucent green wall around the region.
You can generally activate the Workshop crafting menu from anywhere within the Workshop's area of influence: hold down whatever key, mouse button, or controller button you have mapped to "Toggle POV/Workshop". (Look at the Workbench and it'll tell you the key.) Just be sure you're not looking at another item, like a container, when you hold down the key, or the Workshop menu might not appear.
Note that if you're in the Workshop menu and have selected an item to create, you won't be able to Scrap items you find. Press Tab to break out of item creation mode and back into the main Workshop menu.
Workbench inventory sharing
Components in the Workshop inventory are available for Workshop and Workbench use within that Workshop's entire area of influence.
By default it's not shared between Workbenches in different settlements. As discussed in this question, you can set up a supply line to share resources between settlements.
Leftover components do not disappear
Many people believe that when the Workshop automatically breaks down a junk item to get a specific components, the other components are lost.
While this is a widespread belief (and for good reasons), it's not true. Details are in the accepted answer to the question What happens to extra materials from salvaged junk?
In researching the related question How can I break items down into their components, I saw a complaint on Reddit to the effect that the extra, leftover materials get destroyed.
Why this is a common belief
Many people seem to believe this, and there's a good reason for that: it's confusing. And it's confusing for two reasons:
- The composite item is removed from your character's inventory, but the leftover components are placed in the Workshop's inventory.
- There's sometimes a delay before the leftover components reappear. If you want to speed up the process: go into Workshop mode, try to build something, and then exit. This appears to force the Workshop to update its inventory contents list.
Go, science!
I settled the question with science. I've tested this with various items a dozen or so times using the Armor Workbench, the Weapons Workbench, and the Workshop crafting menu.
The leftover components have always (so far) appeared in the Workshop inventory, but there's sometimes a delay.
Here's one of the experiments:
Experiment
- Remove all Lead from your inventory and the Workshop's inventory.
- Add two pencils to your inventory; each pencil has 1 Lead and 1 Wood.
- Go into the Wood Structure crafting menu—not to craft anything but just to check your Wood component count.
- Craft a Radiation Emitter, which among other things requires 2 Lead.
By creating the Emitter, we force the Workbench to dismantle the pencils, which liberates 2 Wood and 2 Lead.
If our Wood component count goes down, wood has been lost/destroyed. If it stays the same, our 2 "potential" units of Wood have simply been converted into 2 actual units of Wood.
Result
Our Wood component count remains unchanged. So unneeded materials are retained, just as though you'd broken the item down yourself.
Best Answer
Spoiler alert! This answer includes items that are not available when you start the game and are only unlocked later. I couldn't get the spoiler markdown tag to work with headings and lists, so the unlockable items are not hidden from view - you have been warned!
This is the complete contents of the workshop menu as I see it at the time of writing. There are some locked items which I haven't unlocked yet, so they aren't listed below. Please feel free to edit this answer to add things I missed.
Structures
Structures > Wood
Structures > Wood > Prefabs
Structures > Wood > Floors
Structures > Wood > Walls
Structures > Wood > Roofs
Structures > Wood > Stairs
Structures > Wood > Miscellaneous
Structures > Metal > Prefabs
Structures > Metal > Walls
Structures > Metal > Roofs
Structures > Doors
Structures > Fences
Structures > Fences > Junk Fence
Structures > Fences > Picket Fence
[picket fences are unlocked by finding a magazine]
Structures > Fences > Wire Fence
Structures > Miscellaneous
Furniture
Furniture > Beds
Furniture > Chairs
Furniture > Containers
Furniture > Shelves
Furniture > Tables
Furniture > Miscellaneous
Decorations
Decorations > Floor Coverings
Decoration > Wall Decorations
Decoration > Wall Decorations > Flags
Decoration > Wall Decorations > Mounted Creatures
Decoration > Wall Decorations > Paintings
Decoration > Wall Decorations > Signs
Decoration > Wall Decorations > Miscellaneous
Decorations > Miscellaneous
Power
Power > Generators
Power > Connectors & Switches
Power > Lights
Power > Miscellaneous
Defense
Defense > Guard Posts
Defense > Turrets
Defense > Traps
Resources
Resources > Water
Resources > Food
Resources > Miscellaneous
Stores
[all stores require Local Leader perk rank 2]
Stores > Trader
Stores > Armor
Stores > Weapons
Stores > Food and Drink
Stores > Clinic
Stores > Clothing
Crafting
[all crafting stations and workbenches require Local Leader perk rank 2]