Well BillC, you’re a lucky fella to be able to see mig-SC (short-circuiting) digging into the base metal. Mig-SC really is no different in principle then any other weld joining process actually. It relies on and requires sufficient electric current to melt both the wire and a proper amount of base metal. Proper amount of base metal directly relates to the amount of dilution (mixing) of filler material and base metal. Since Mig-SC’s arc transfer mode is of low heat input, it inherently produces a small, fast-freezing weld puddle characterized by its shallow depth of fusion into the base metal. You would see this in an etched cross-section of weld.
As was similarly stated in another reply, welding is a process that produces coalescence of metals (base metal & filler metal) by heating them to a welding temperature. The process of brazing is a joining process where the base metal doesn’t melt so there’s no dilution of filler and base metal.
Your concern for asking the question is correct though, if you have no degree of dilution, which is what’s called “cold-lap” with mig-SC, there’s no fusion!
I have to say I’m a little disappointed in your instructor for teaching a welding process without first explaining the principles of it. This seems to be the trend today in many training centers I’ve noticed, teach the student to pass a test but don’t teach the student much about “welding”.
There's no such thing as a welding problem, there are only welding puzzles of assorted sizes!