It isn't hard to program a gpt. You can do it in a weekend with a few hundred lines of python. The code is pretty simple. The math is not particularly high level.

The complexity and scale with LLMs come from the amount of training data used, not some kind of black magic in the programming.

> It’s all just physics and chemistry, and those are well understood.

Not really. We cannot model physics and chemistry to a level which allows us to accurately predict a humans action (even a tiny time-step into the future)

This is vastly different to an LLM, where the model is the model (for a lack of better phrasing).