After the basics things start to get more interesting from a design perspective.
Portrait and landscape orientation
One way of looking at the portrait and landscape orientation is they are nothing more than a change in aspect ratio. But especially with a phone the design is not primarily an issue or a narrower or wider screen, but how the device itself is held.
One and two handed use
Phones are primarily held one handed. It is not unusual to find yourself with only one hand to operate your phone. Be it carrying the shopping or holding onto a rail on the train. The entire interface of a phone, including being able to type text needs to be possible one handed. The only position where you can safely hold the phone in a firm grip and still reach the majority of the screen is when the device is held in portrait.
When held in landscape the phone cannot be firmly held and operated. Landscape on a phone is for two handed operation. On tablets one handed operation is not a design consideration due to the size and weight of the device. However hand position is an important consideration and means key buttons in tablet apps need to close to the edge of the screens where thumbs can comfortably press them.