Initially, each settlement's raw materials are its own and are not shared with other settlements. However, there is a perk called Local Leader. Getting the first rank of Local Leader that enables you to assign one settler to act as a Provisioner, linking up your settlements with supply lines and allows them to share resources. The second rank of Local Leader allows you to build vendor stations, making it easier to buy/sell stuff. These vendors also sell stuff to settlers, earning you caps over time. These caps can be picked up in the workshop inventory, under miscellaneous.
When you are scrapping stuff, take note of its color. If its yellow, scrap it as it is a junk item. If its green, that item is usable by the settlement or settlers. Settlers need chairs, tables, radios, televisions, paintings, etc.. for their happiness.
Always build your settlements with defense in mind. This is because settlements can be attacked by anything from raiders to really tough super mutants and even the occasional death claw. And if given the chance, attackers will first destroy your turrets and power generators (More advanced turrets require power and the AI is smart enough to know taking out generators also takes out all your advanced turrets). Next, they will destroy your food crops, water sources and kill your settlers. Thankfully, everything can be repaired as long as you have the necessary raw materials. And as long as you have a working and powered radio beacon, new settlers will slowly come in over time.
I can't say where you should build your defenses as it varies depending on how you lay out your settlements. For myself, I usually start by planning the layout of the settlement itself. Where possible, food crops, water supply and power generators should be in the center. I will usually have 2-3 turrets guarding these as the final line of my settlement's defense. Next, I usually build a perimeter fence big enough to circle the resources and buildings I want to protect. Even something as simple as a wire fence is enough block attackers and channel them towards the settlement's entrance.
This allows me to concentrate most of my defenses in one location, usually a bottleneck where my turrets and settlers can focus all their firepower on. Next, I will scatter turrets around the perimeter, pointing them outwards to whittle down attacker's numbers as they make their way towards the entrance.
Oh, and arm your settlers with better weapons and 1 ammo (as long as they have that 1 ammo, their weapons will work). You can also build defense posts and assign a settler to them on guard duty (they will patrol between the defense posts). Also if you have built an alert siren, your settlers are smart enough to toggle it on when they see an attack incoming. This siren makes all settlers stop whatever they were doing and draw their weapons, making them ready to repel any attacks.
