It’s possible to take still photographs while recording a video. While filming, tap the smaller white button on the toolbar. These images will then be saved to the Photos app.
It’s also possible to zoom in while filming. Before or during recording, pinch outwards on the screen to zoom in. Zoom out again by pinching your fingers together on the screen.
Alternatively, if you have a multi-camera device, tap the 0.5x, 1x, or 2x buttons at the bottom of the display to immediately switch to that zoom level.
Note that extreme close-ups will cause the video to look grainy when played back later.