I made a little TUI last month for searching within a channel! It supports before: / after:, fuzzy/exact/regex matching, lets you order by upload date/views/duration, lets you search over just a video's titles or descriptions, etc: https://github.com/nolenroyalty/yt-browse

The vast majority of my youtube watching is "go to a specific channel and try to find a certain kind of video" so it drives me nuts that youtube channel search is so bad (and afaik you can't search a channel on mobile?). I end up using my tool to find a bunch of videos and get them into my history to watch on my ipad.

n.b. my tool downloads all video metadata for a channel and then searches over it locally, so it's pretty slow the first time you search a channel (results are cached for 24 hours though).