Extending the size of the video shot on iPhone
Unfortunately, the screen is showing the whole frame. That is all that your camera recorded. It did not record anything to the left or right of what you see.

On YouTube, where many people shoot their videos in a vertical format as you did, it is common to enlarge the video to the full frame width - which will cut off the top and bottom of the video - and use that as a background image. Then place your regular-size vertical video over the top of it. I used iMovie in the past and I think its picture-in-picture feature could accomplish this, but I'm not sure.