When it comes to mobile applications the first thing we require from them is functionality. However when we choose among numerous apps with similar functions on the App Store or Google Play Market, we tend to install the ones that have more beautiful design. Taking this into account it can be said that UI & UX are no less important than app functionality. 

Each year new design trends appear and this has to do with the devices which displays keep changing shape and size (read here https://vocal.media/01/top-10-mobile-app-design-trends-in-2019). The design becomes challenging as it needs to remail simple, appealing and yet user-friendly. As new technologies appear (like AR, VR) the design should be changed and adapted. After all, nobody wants to use apps that look outdated. 

Let’s say you create the design for the iPhone and you need to make it adapted to any screen size or iOS version.  That’s not an easy task, yet it is possible to do this. It is also necessary to take into account the status bar differences on old and new iPhones. The color scheme plays a crucial role as well. Design requires holistic knowledge and as devices change quickly, the designer needs to keep pace with all trends. To get some fresh information about current design trends, read this publication.