I may be wrong in my understanding but I am gonna throw this out there anyway because it is based off my past experiences.
And I am only trying to help you understand what I believe to be true.
When a creator creates something for a kit, I think they can only use build/cas of items that are already in the base game since kits have to be compatable with every expansion, game, and stuff pack. If they want to create a totally completely 100% new item, never in the game before, I think they have to have consent first if it's to be sold and not CC for personal use??? Creating kit content for the game is not the same thing as having CC or MOD's. They are entirely different creatures.
Then there's the coding for each item like the bath tub has to be the same as with all the other bathtub/showers in the game. The actual picture that you see can be altered but only within the parameters already set with that object. If you have never created an build/cas item for yourself for your game, I encourage you to give it a try!
It will enlighten you and you will then really understand what these creators really have to do. I used to create my own stuff based off the items in the base game build/cas only. Using cas/build items from specific packs would cause issues with my game if I wasn't careful with the coding for ...hmmmm lets say, the fridges or washer/dryer which is only availble if you have that pack. It's been over 4 years since I have used cc - never used mod's. edit - or created cc for my game. (eyes not so good anymore)
I'm pretty sure you've confused something here. When they say they can only use stuff that already exists as a base, they mean, like...we can make bracelets. The game has bracelets. We cannot make bellybutton rings, the game does not have those. We can make stoves, the game has stoves. We cannot make air fryers. The game doesn't have air fryers.
They don't literally mean "We have to take an existing thing into blender and smush it around into something new".
I'm not sure if the tuning part is even correct, there are functional objects in other kits that have their own unique tuning files, despite being existing object types. Many showers, TVs, etc have unique tunings per-object to handle things like power usage, effectiveness, available upgrades, special VFX, etc. The comfy gamer candles for example have their own unique tuning that determines their VFX and a broadcaster that gives off a buff when you're near them.
