zyBooks are designed to work in HTML5-compatible browsers. That includes nearly any popular browser: Chrome, Firefox, Safari, Edge, and more. And it includes most popular devices: desktop computers, laptops, tablets, and smartphones.
With that said, a smartphone is quite small for the learning experience that zyBooks target. Thus, we say a smartphone is "OK in a crunch", but you should plan to do most of your learning on a desktop, laptop, and/or tablet.
Because HTML5 is so powerful and provides nearly the same experience as an app, we don't have current plans to develop an iOS or Android app, using our engineering resources to build other features to help instructors and students.
See our system requirements here: https://www.zybooks.com/system-requirements/