How do I let the user customize a specific portion of model in React Three Fiber?

Question

I am still learning React Three Fiber and after following several tutorials covering te basics, I am still stuck on how to achieve the following:

Let's say my website displays a simple model, such as a chair. I know how to let the user customize colors, textures and scaling, but what if I want to let the user enter the length of the chair's legs, for example? How do I change the Y scaling of the legs only, and nothing else?

Another similar question: Let's say the model has a round hole somewhere. How do I let the user specify the diameter of this hole?

Any suggestions or ideas would be greatly appreciated.

I have followed several tutorials and I was able to change the entire model's scaling, but not one specific part of the model.