We are digital librarians. Among us are represented the various reasons to keep data -- legal requirements, competitive requirements, uncertainty of permanence of cloud services, distaste for transmitting your data externally (e.g. government or corporate espionage), cultural and familial archivists, internet collapse preppers, and people who do it themselves so they're sure it's done right. Everyone has their reasons for curating the data they have decided to keep (either forever or For A Damn Long Time (tm) ). Along the way we have sought out like-minded individuals to exchange strategies, war stories, and cautionary tales of failures.
3. How to sign in with youtube-dl?
I'm not sure if this will work, but you can use --cookies cookies.txt alongside with a cookies.txt file (you can get it with cookies.txt add on). It works for downloading private playlists. So it may also work for age restricted videos.