I built something similar to this when I was first learning to program. Except mine lets you perform a one-time search for videos containing keywords, similar to how you would normally search YouTube. So there are no notifications or anything.
I don’t know if it even works anymore and I’m sure the code is atrocious. But I remember that I would just scrape YouTube pages for video IDs, and then use an API that returned video captions for a given ID [1]. I could see how OP would do something similar.
Nice job on PhraseFinder, @SteveDR! I am indeed doing something similar for my app. The user enters the YouTube Channel URL, the keywords and the date range, then I perform an on-the-fly search to find the matches.
I replied to @lewisjoe's comment above this one with some more details about tricks I'm employing to make searches fast.
https://phrasefinder.net
I don’t know if it even works anymore and I’m sure the code is atrocious. But I remember that I would just scrape YouTube pages for video IDs, and then use an API that returned video captions for a given ID [1]. I could see how OP would do something similar.
[1] https://pypi.org/project/youtube-transcript-api/