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16 pixels by 16 pixels is a common resolution for assets matching a low-res pixel art aesthetic. Although the assets may not be specifically 16x16, they are designed to work in a graphical setting where objects are around that size.

Resources relating to or for building Graphical User Interfaces (Commonly called GUIs)

Unity is a game engine for 2D, 3D, AR and more

Unreal Engine is a 3D game creation tool