The short version-
Carbon has several unique properties, 2 most important to welding are it's electrically conductive and unlike damn near any other element, SHRINKS rather than expands as it gets hotter.
Carbon is a repair weldor's best friend and a machine tool operator's worst enemy because a weldor worth his stinger can build repairs that eliminate most of the machining.
By shaping the carbon to act as a mold, a weldor can easily build a tooth back onto a gear rather than build a pile and let a machine tool operator carve it to the shape of the tooth.
Carbon is easy to shape, and can be clamped into place.
On the down side, carbon is single use, and filthy to work with.
BTW Nola, welding is something you NEVER stop learning. I been burning up metal for 50 years and I'm still learning. Damn little of the learning was done in any school. A weldor has the job completed in his head before the machine is turned on. A welding machine operator is just somebody behind a mask repeating day after day.
From what you've posted you got the makings of a weldor inside you, a few thousand hours of practice you'll be a mechanic.