It was a mix of techniques, this article has been in progress for quite a while :). The first part was modeling all the relationships for an accurate beam engine in onshape (great cad tool). Then, I built a custom exporter to cleanly export it to three.js. This ensured physical realism for the hero of the article. All of these are linked from the article.

For the figures, I had prototyped some by hand, and it was slow going. Fable was the first LLM I found that was actually able to understand and model mechanical linkages correctly enough to be useful. I built a style guide, and iterated many times on each figure and animation until we arrived at something useful. There are also quite a few backing tests to ensure physical correctness of the animations (as far as reasonably possible :).