I know for a fact that Alec either transcribes those videos himself (not hard to do from a script he already writes for himself), or gets it done by someone els

I know for a fact that Alec either transcribes those videos himself (not hard to do from a script he already writes for himself), or gets it done by someone else before uploading the video. I'm any case, they're always uploaded at the same time as the main video. Also, YouTube recently removed the ability for individual users to contribute to captions of creators. There was a thread on HN about it.