And let me build better items (furniture, prefabs etc) for my settlements? Or am I stuck with what I can craft at the beginning of the game?

If you find the several Picket Fences magazines you′ll get more items, but not that many. Because of this is important to know what can be build and what belongs to the world when scraping the old stuff on settlements, as some of that stuff is not buildable and unique.

I'm glad I didn't scrap the Patio furniture in Sanctuary. It looks pretty awesome in my house I built. Put up a nice 2 story house with vaulted ceiling and many patio's though uh the roof could use a little work keeping more of the rain out.

You get some via magazines. Picket Fances series is focused on adding new items, but I think some other mags do as well (an issue of Wasteland Survivor guide gave me some new items too).