Help me understand supply lines

Every time I think I have supply lines figured out something else comes up. Can somebody help me explain exactly what is shared between settlements and when?

It seems that if there is junk in one workshop and I want to build something at another settlement with a supply line to that Workshop it will scrap the junk to help me build what I need but the junk itself is not accessible. So in other words if I have silver stored in a workbench and try to build something that requires it in a different settlement it will be scrapped but I cannot pull the silver out from that attached settlement. Is this accurate?

What about crafting? If I want to make a gun mod or a something at a food station and the stuff I need is in another workbench that is connected will I have access to that or do I need to have it all together?