You can download Udemy videos on your Smartphone or Tablet for offline viewing by using the Udemy app. This app is available on both IoS and Android platforms.
This is a great help when I travel. I can download the complete course or some specific lectures that I can watch on the plane, where I do not have a network connection.
Please note that you can only download videos in this app. Practice activities, such as quizzes and practice exercises, can not be downloaded or completed offline.
More details are available at these two links:
Downloading Courses on Android