Main resources I use:

1-Settlement waste materials (trees, burnt cars etc...)

2-Scrapping weapons

3-Buying shipments

4-Scrapping Junk

Do you have any other method?

Do you take junk from everywhere in order to scrap?

Do you scrap junk periodically to get underparts?

Are workshops chained together in order of storing weapons, bottlecap and armor? Is it related if they are supply lined?

Do settlers steal anything I leave behind (power armor, weapons in workshop etc..)?

Why does Preston continue giving defend quests from same settlements?Is it normal?

pipe rifles yield copper, wood and steel

Well, I scrapped everything in every single settlement so steel and wood have never been a big deal for me. For copper I tend to collect broken light bulbs and vacuum tubes.. they are light and numerous.

I go the store and buy it. I have far too many caps anyway.

this. and changing the way you build up places.

Your best bet for steel is those annoying tin can traps that rattle when you walk into them. Cans are the best value to weight item you can collect for steel. I hardly ever scrap weapons. I will scrap armor/clothing for leather though.

Most of my materials come from scrapping out settlements I don't intend to ever build anything at. I do buy shipments from time to time when I am out of the rarer/hard to find stuff.

I have a list I keep in my head of junk items I pick up when ever I see them. Telephones, microscopes, fuses, glue, oil cans, gas cans, vacuum tubes, typewriters, hot plates, desk fans, etc. Generally anything with gears, screws, oil, adhesive, nuclear material, aluminum, fiberglass, fiber optics, circuits, gold, silver, etc.

When i played the game and required those items i would put my building on hold and acquire them while I travelled.

If you put collected junk items in connected workshops their component materials are available throughout the network.

I would kill, loot and subsequently barter the loot for shipments and also barter for junk items that contained the required materials and put them in the nearest workbench for later use.

Items like guns and armour aren't shareable through the connected network.

Being on a pc I resisted the temptation of using the console to put shipments in my inventory.

When i first started playing I would tend to hoard guns and armour but after a while realised it was pointless and spent some time doing a cleanup.

I had too much to be able to sell in a short time so I sold what i could and had a big clean up day, scrapping everything I didn't want.

I put some of the rare items that i never use in storage at home plate.

From then on I stopped hoarding non junk items and disposed of them immediately.

Outfitting settlers, bartering for ammo and so on.

With regards to settlers, yes they can use things that are available to them.

If you periodically go around and check their inventory you may find some that have looted and borrowed things from storage.

They can also use power armour if they have access to power cores.

I have placed fusion cored pa suits at all settlements and sometimes the settlers will jump into them when an attack occurs.

I have found they are more likely to use them if the pa is in a pa workbench and near to where they are stationed.

Most don't stay in it forever, especially if they have duties requiring them to get out of it.

Also if you talk to them the only option is to tell them to get out of the armour.

I do have one Provisioner, though that jumped into a pa suit during an attack and continually uses it as they travel between Red Rocket and Abernathy. Provisioners never sleep.

I have sunshine trading set up as a dedicated scrap facility. Have only 18 people so far. 3 on food and the rest on scrap benches. I've never put anything in the workbench apart from the original raw materials made when scrapping all the trees and stuff but the workbench has a massive list of both junk and raw components that the plebs have scrounged up. Their efforts keep the scrap rolling in. That and the fact I don't leave anything behind that's not nailed down. If it is nailed down I come back later with a crowbar and my pack mule, um I mean Strong ?? And I buy shipments and pretty much all the useful scrap that shops have. Not much else to spend caps on later in the game?

Rather than scrap guns and armor, sell them and buy junk from vendors. You'll get more material out of spending caps than the pittance you get from scrapping.