Metal is fabrication is the process of assembling a metal structure from individual parts made from standard metal stock which can include, sheet, plate, bar and tube (as well as extrusions in the case of aluminium).
This can include various processes used to cut the metal to size, shape it and join it such as :
cold forming, including open die forming and pressing
drilling and punching
welding brazing and soldering
Fabrication also tends to imply that at least some of the parts are (more or less) permanently joined and so it tends to be closely associated with welding processes.