PC and mobile game UIs are different mainly because of screen size and controls.
PC UIs are made for mouse and keyboard, so they can have smaller buttons, more menus, more information on screen, hotkeys, drag-and-drop systems, and overall more complex layouts. Since players usually sit longer at a PC, the UI focuses more on efficiency and giving access to a lot of features at once.
Mobile UIs are designed for touch controls and smaller screens, so everything has to be bigger, cleaner, and easier to tap. Mobile games usually simplify menus, reduce clutter, and keep only the most important things visible during gameplay. They also focus more on quick access and short play sessions.
Basically:
PC UI = more detailed and information-heavy
Mobile UI = simpler, cleaner, and touch-friendly