youtube-dl is an opensource command line tool to download video or audio from online video streaming services.
Videos downloaded in mkv or webm extensions can be played by VLC Media player in all major devices and operating systems including iPhone, Android devices.
Tool website: https://youtube-dl.org/
This gist shows the example commands to use the tool and doesn't support or encourage piracy or violation of copyrights of the online streaming service or the author of the content