Kitbashing can be as simple as putting an out-of-the-box anti-aircraft gun kit on the back of an out-of-the-box flatbed truck kit with few or no mods to either, or as complicated as using hundreds of different kits to build & detail one model, like the Star Wars movie Death Stars...
Scratchbuilding is making your own parts from raw materials, much like Hawk's scratch-baking scenario.. As simple as stretching a piece of sprue into a radio antenna to as complicated as creating an entirely original model with no pre-fabricated parts anywhere (If there are, they've been modified), made from various materials like styrene, ABS, epoxy, resin, wood, metal, paper, and by carving, stretching, molding, vacuum-forming, thermal-forming, accompanied by blood, sweat, & tears,etc and all stuff in-between...
Both scratchbuilding and kitbashing are techniques made up of skill-sets that are required to be a "Modeler", are nice-to-have"/optional for a "Model Builder", & beyond hope for an "Assembler"...
For instance:
I needed a nose-castoring set-up to build my "Ralstion Trainer" SNJ... No kit exists of this unique Navy bird, so I had to scratchbuild the castoring system from styrene rod and a Kingfisher tailwheel, then mount it on the Monogram SNJ kit..