@EdChdX  
Yes, and no, you can get the metal from what you’re welding, it does make the metal from around the weld thinner after you melt and flow it, but that’s why you don’t see welders with a big pile of spare rebar or whatever where they’re working. In some forms of welding, namely tig welding you do have to have a hand held feeding rod, but in Mig, or Stick welding you typically work with thicker materials which you can use to do the welding for you.
 
I will admit that you would prolly use a Tig welder for airplane applications due to aluminum being a very difficult substance to work with, but that’s artistic license for you.
 
Edit: Source: I work on a farm in my spare time, and we have to do welding quite often.

with just fire or arc without another piece of metal for melting, it’s cutting through metal, not welding…