Yeah, the reference viewer saved my bacon many times. It basically shows all files the chosen one is being referenced in (to the left) and all the files it references (to the right). Child classes are also located to the right.
Submaps are just what they sound like - portions of the main Island map, which is separated into 25 surface areas (from A1 to E5) and caves, so as not to keep too much information loaded at once. This is very obvious with caves: their entrances viewed from afar are untextured and allow you to see through the world, but entering them causes a “lag spike” as they are loaded in.
From my understanding, it is indeed the “attached component class” that matters for harvesting. Unfortunately, I haven’t been able to find an option to remap components or foliage (if there is one), and editing these objects will not work for a normal mod as you’d have to edit the map to replace them. It should work for TC, but in that case it would be easier to edit the HarvestComponents than each foliage (there are many more variants).
The “Remap Resource Component” under PrimalGameData is worth looking into, but I haven’t been able to figure out how it works yet.