RetroArch (RA)

RetroArch is a free, open-source front-end that loads individual emulators as interchangeable modules called cores, giving one consistent interface, control scheme, and feature set (save states, shaders, run-ahead) across dozens of systems. It is the backbone of a huge share of handheld emulation setups. It is powerful and, for newcomers, a little intimidating — which is why some devices ship simpler custom front-ends layered on top. It manages emulators, not game files, which remain your responsibility to own legally.