It's a very large context window, but it is compressed down a lot. I don't know every line of insert your PL of choice's standard library, but I do know a lot of it with many different excerpts from the documentation, relevant experiences where I used this over that, or edge cases/bugs that one might fall into. Add to it all the domain knowledge for the given project, with explicit knowledge of how the clients will use the product, etc, but even stuff like what might your colleague react to to this approach vs another.

And all this can be novelly combined and reasoned with to come up with new stuff to put into the "context window", and it can be dynamically extended at any point (e.g. you recall something similar during a thought train and "bring it into context").

And all this was only the current task-specific window, which lives inside the sum total of your human experience window.