Although I don't use YT downloaders often, the only ones I've used in recent years have been online ones, and only from a mobile device. Most of them seem to be infected to the bone with malware making them unusable.
One thing for the few that do work, it's always intrigued me how they work from the perspective of being able to provide a downloadable file in far less time than real time old fashioned screen scraping, typically it's only a few seconds. I'm wondering how this works, I'd have thought the stream itself would be throttled from Youtube. I have thoughts of multithreaded concurrent downloads from a server farm, stitching together the content. Or maybe it really is just a simple unthrottled stream.
For the non web-based solutions, do they also work faster than real time, or are they screen scrapers?
I don't know why YT doesn't offer downloadable content in most jurisdictions, I would happily pay for that as a subscription based option, especially if it avoids ads too. The mentality that we're always 100% internet connected does not apply in one of the most common use cases for watching content, i.e., while travelling. I believe a few areas have Youtube Red but I don't know what that offers, is it all content or is it limited?