Supply routes are great when a settlement is just starting out to get things off the ground, but what is specifically transferred along them? I know anything on the Junk, Aid and Mods tabs are transferred, and Fusion cores and Power Armor pieces aren't. Does that mean weapons, armor and ammo aren't transferred at all?
And what about Food and Water resources? Can I build a farming community with a surplus of food and use that to feed a merchant-only settlement, or are local crops always required? Ditto water.
Best Answer
The current accepted answer seems a bit unclear to me, and may actually be inaccurate. I'll try to explain better, by my understanding, here.
The only remote resources that are available to the player, in settlements that are linked via supply lines, are those which can be used for crafting. Even then, those resources can't simply be collected from the Workshop's inventory. The only way to access them is by using them in the crafting process.
This means that you can't drink Purified Water that's stored at a remote location, but you can use it to create Vegetable Starch. You won't be able to take a Blood Pack out of a connected workshop and put it in your inventory to heal yourself, but you can make a Stimpack with it. Etc.
This also means that not all Aid items will be available across a supply line. Meat that's already been cooked, drinks that have been made, or drugs that have been blended will most likely not be accessible because most of these items aren't used up by any recipes.
Mods are available to be added to a weapon if they are in a remote settlement's workshop.
Weapons and Armor will not automatically be broken down into components when crafting, so will not transfer via supply line.
Furniture, Walls, Turrets, Shops and other craftable settlement stuff will not transfer via supply line, with the exception of crops. When one stores a Mutfruit tree or a Tato plant, it simply shows up in the workshop as a Mutfruit or a Tato, and those items transfer for the purposes of crafting.
If junk in a remote, connected workshop is automatically broken down, the leftover components are deposited into the local workshop (the one the player is crafting from). (This needs further verification from somebody else - only 95% sure on this).
Nukapedia claims that food and water will be shared across the line, for consumption by settlers, even though the UI might not reflect this. More testing is still required on this, but from what I can tell, food and water is indeed transferred along supply line, but not in an obvious way. My observations:
So it appears as if Food and Water is transferred via supply line, but not instantaneously like crafting components. Further testing is required to determine the details of how it is transferred; also if this is a total misreading of observation or not.
TL;DR:
*Testing Needed
Please comment/edit if you can help answer any of these, to further clarify how supply lines actually work.