You can download MP4 files of videos that you’ve uploaded to YouTube in either or 720p or 360p, depending on the video size. You can’t download other people’s YouTube videos, but you can watch videos offline in the YouTube app if you have a YouTube Premium membership.
Learn more: https://support.google.com/youtube/answer/56100
Save videos offline with YouTube Premium: https://support.google.com/youtube/answer/6308116
source