You can download the materials, including videos, to any device. I do not have an iPhone, but I'm sure the process is the same. I just go to the study planner on my Android, click download video, and it downloads to my phone for offline viewing.
I realize this is a 3 year old thread, but had the same issue with downloading videos to my iphone so I could view them without a good cell/internet signal. My workaround if anyone else needs it: Download the videos to your computer, upload them to Google Drive. Open Google drive on phone, hit the three dots next to the video and select make video available offline. That downloads it to your phone to watch offline.