A few assumptions:
For language learning, the actual content of the video is not the main factor. We want videos that are interesting enough to keep you watching, but also have high quality audio, useful vocabulary and match your proficiency level. If you can follow the contents of the video and think you can learn something from it, you'll probably keep watching.
Skipping a video is a signal that this video is not for you, in terms of difficulty or content or both. When selecting the next video, videos with similar tags get downranked. Watching over 10 seconds of a video is a positive signal, and the system will show you more of that kind. You can see the positive and negative tags at the bottom once you start making choices.
The basis for this are over 1000 curated YT channels, so we can generally assume a decent quality for most videos.
