How do you transfer youtube videos on instagram? Iphone please help thank you.?

[LA Kings]

Let's say I want to put up a video that I have on youtube but I want it to be transferred to my photos section of my Iphone. I tried many ways to get it to my photos sections so then I could be able to transfer it to my instagram, but it never worked the video was in the videos section and their is no option to select it from there if anyone knows please help thank you.
I still can't do to the fact that the video is in the movies section there is no way I can get it in the section where I can upload it to instagram. I know it sounds confusing, but if you have an iphone I think you will understand.