I disagree we understand LLMs, your view is quite reductionist. It's like saying we understand human brain because we understand physics and evolution, or that we understand some program because it's just a binary code in a known instruction set.

I think Dennett's theory of intentionality (see https://en.wikipedia.org/wiki/Intentional_stance) applies here. We do understand LLMs from physical stance (ML algorithm and inference), but we don't fully understand them from design stance (it's internal workings have been evolved so it's hard to tell the functional units) and from the intentional stance they are a complete mystery.

And I talk about obstacles to this understanding elsewhere in this thread.