I don't know the specifics of this particular tool, I assume it's at most using a couple of passes of (some frontier model with specific system prompt + custom tools, for example code-specific rag + some form of "summarize"). By at most I mean "probably isn't doing anything crazier than that".

But it seems to be producing docs that are better than I tend to see with basic "summarize this repo for me"-style prompts, which is what I usually use on a first pass.