So far, we have built a dozen or more apps of various complexity. Most were designed and tested on a phone.
Of course, in the real world, our apps need to work well on any device and must be able to handle what happens when in either portrait or landscape view (on all devices).
Furthermore, it is often not enough for our apps to just work and look OK on different devices. Often, our apps will need to behave differently and appear with significantly different UI based on whether the device is a phone, a tablet, or landscape/portrait orientation.
Note
Android supports apps for large screen TVs, smart watches via the Wear API, virtual and augmented reality, as well as "things" for the Internet of Things. We will not be covering the latter two aspects in this book but, by the end of it, it is the author's guess that you will be sufficiently prepared to venture into these topics should you choose to.
Look at this screenshot of the BBC news app running on an Android phone in portrait...