UI (User Interface) design focuses on the look and feel of the app, including layout, colors, and typography. UX (User Experience) design is broader and involves the overall experience a user has with the app, including usability, accessibility, and how intuitive the app is to use.

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https://appdevelopments.mystrikingly.com/blog/user-interface-design-principles-for-mobile-app-development

https://sites.google.com/view/appdevelopmentco/blogs/user-interface-design-principles-for-mobile-app-development