Currently, YouTube does not reliably include spoken word/transcript matches in search results. You would have to manually open each individual video in a browser tab, open the transcript, hit cmd/ctrl+f and search for the words you're interested in. And if you need to search tens/hundreds/thousands of videos, that would take a very long time.

VideoMentions Search allows you to specify the channel URL, keywords and date range, then does all that hard work for you, quickly locating all videos that contain those keywords.

I hope that's helpful. Thanks for checking out my project!