From my experience they don't, if I want say screws from items for wepeans mods I have to scrap them on the floor myself, the desk fans hotplates etc just stay whole in the workbenches when you store them, unless I'm missing something obvious on what to do, please explain?

The item remains until you build something requiring screws.

Then it is broken down.

They stay whole, yes.

But when you want to build or mod something, the items are broken down then. Best test is something that requires a rarer component, either adhesive (weapon/armour mods) or copper (power lines). Drop some duct tape off for example, mod something and then look at the junk store, the duct tape will be gone.

If you store an item in the workbench, it doesn't get broken to components on that instant. If you however try to craft in, say weapon workbench in the same settlement, and need screws, and your workbench inventory doesn't have any screw items in, it uses the junk stored instead, like the desk fan if it has any. The listing you get when first hitting the "craft" button show's what exactly is broken down for the items. Have had fun from crafting armor mods from cigarettes (cloth) for example

So why doesn't this work when I go to mod weapons, I have desk fans and hotplates in the junk store yet, on the weapons modding screen I have no screws so it doesn't work..

You can also test it on an workbench not in an trade network and bring some stuff to it.
Note that the leftover apears to be missing but you can still mod with it and it shows up next time you visit

Sounds like a silly question, but are you using a weapons bench in the same settlement that houses your workbench ?

Thanks, now I think I get it

Yeah, it only works if the stored stuff is in the same settlement as the weapon bench you're trying to use. I think even with the supply lines features that's so, for whole item parts.

It only works with connected stores/settlements. If you store stuff I your workbench in sanctuary, onky the workbenches in the sanctuary see it. If you connect sanctuary and red rocket station with supply route, you can peruse the stuff also in red rocket workstations (but not withdraw the items from the workbench as they still reside in sanctuary). And it only peruses the items you have in your inventory if you use some workbench "out there", outside your settlements.

Oh ok, then it does work whole items with the supply route/separate settlements? I thought it had to be broken down for that aspect for some reason.

I think it works like this. All items are shared between all stations and workstation in a settlement. Junk will stay in your workstation until you want to make something via building mode or in another station at which point it will open a window telling you what items will be scrapped to get the component. So if you needed a screw to make a weapon via the weapon station, it will bring up a window telling you it will take a screw from a toy car stored in the workstation which will be broken down/scrapped to get the screw.

Yep, works for whole items too.

You can't, for example, store all of your junk in Sanctuary, then go to the Red Rocket work bench and browse through the same list of junk, but when you try to build something or mod weapons, armour etc. in Red Rocket , the 'raw materials' (seperate materials and materials from items that need to be broken down too !) from the work bench in Sanctuary will be available to you.

I've done a couple of different tests that prove that junk items do get broken down when crafting, even across supply lines. There are several catches, though. First, items remain stored in the Workbench the they're generated at, and can't be pulled from other workbenches. So you can't pull stuff stored in Sanctuary from Red Rocket's workbench. However, if the two are connected by a supply line and you need supplies from Sanctuary to build something, it will let you, and scrap things if needed. Second, only Junk and Aid items are shared. If you are low on Steel, but have a bunch of Raider armor, you won't be able to build anything because the game won't break down weapons or apparel. Technically, it also doesn't break down Aid items, but they are shared both with other crafting benches in the area and across supply lines.