Generally the repository he was working in, but really it was any application that he had open on his machine. He would remember where words, or portion of words that he needed were, go to them, and copy and paste what he needed.

Just in case you're thinking this: He was not copying large portions of code from stack overflow or anything like that. He was line by line writing code, a few copy and pastes at a time. Often he times would copy and paste single characters to maintain his flow.

Start with a file like: abcde...ABCD..1234.{}()*.... and go from there.

Voice dictation, Shirley.

Stack Overflow, surely

programmers he subcontracted to.

Also explains why he was so fast

OpenAI, surely.