RetroArch is a free, open-source modular frontend interface used to run emulators, game engines, and media players rather than being an individual emulator itself. Developed by Libretro, it acts as a “universal remote” for classic gaming, allowing you to manage your entire library of vintage games from a single, unified interface.
Instead of downloading separate apps for the NES, Sega Genesis, and PlayStation, you run them all inside the Official RetroArch Platform using modular plugins called “Cores”. How RetroArch Works: Cores and Content Intro To Retroarch