This is done by mistake on iOS 10, you use iOS 11 can try and only apply to devices with dual cameras.
First, you open the camera app, as usual, zoom up to the maximum and keep your hand on the screen.
Then you use another finger to touch a point on the screen for focus and immediately swipe to the video mode while the finger holding the zoom position remains in place.
At this point, you can comfortably zoom to 10X when shooting video.
The text is a bit complicated, you can watch the video below for easy implementation.