In the latest update of the Noteshelf app, the Zoom Box icon has been shifted from the main toolbar to the More menu.
To use the zoom box, follow these steps:
1. Inside a notebook, tap on the More option
2. Turn on the toggle next to Zoom Box
3. Tap on the magnifying glass icon on the bottom right of the page whenever you want to write in the zoom mode.
4. Tap on the three line icon to close the zoom box.