QMK (Quantum Mechanical Keyboard) is the open-source firmware that powers most custom keyboards, enabling deep customization of layout, macros, and lighting. VIA is a browser-based graphical interface that simplifies QMK configuration without coding. Key features include: remapping any key to any function, creating macro sequences for repetitive inputs, setting up layers for compact keyboards (e.g., mapping arrows to IJKL on layer 1), configuring RGB lighting effects, and adjusting debounce and tapping term settings. For keyboards that support it, VIA allows real-time remapping without flashing firmware. QMK also supports advanced features like tap-dance (different actions for tap, hold, and double-tap) and combos (triggering an action by pressing two keys simultaneously).