Why do you need to understand the code? Ask the LLM, it'll already do a better job than humans can.

Much better at satisfying formal specifications (what they were trained for), but not yet good at long term maintenance.

For long term maintenance you need to throw features out / delete code, which requires a deep understanding of the balance between what the code can survive and what is really needed in reality while not growing the code base too much (which neither LLMs nor humans can handle)