By default upon joining a call, your call partner's video feed will be in the main view and your video feed will be in the secondary view. You can toggle between the two by tapping the icon in the bottom right of the minimized feed.
Pinch the screen to zoom the video feed that is in the main view. Keep in mind that you will only be able to zoom in the remote feed if your call partner is using a device that has that functionality.
The zoom level will show in a temporary pop-up on your screen, and your partner will receive a notification like below.
Tap the snapshot button to capture a snapshot from the video feed that is in the main view.
Swapping Video Feeds
The zoom and snapshot buttons control the camera that is in main view.
To swap video feeds between the main and secondary views, tap and hold the swap icon on the bottom right of the secondary view.
Tap the flip camera button to swap between your front- and rear-facing camera.