I'm not sure if there's a way to stalk someone's comments across several videos. However, you can use this to only download all comments per video via yt-dlp, and then search that for a specific user:
yt-dlp --write-comments --no-download URLGOESHERENote that this saves all the data on each comment, including likes, pin/fav status, and the account ID and URL of each commenter. Very handy for archival and search, but not something you'd want to scroll and read through. It also includes the video metadata.
