Comment by gassi

The project that powers the in-browser emulator is EmulatorJS, which use RetroArch's libretro cores compiled to WASM. More info on the process here: https://emulatorjs.org/docs4devs/buildingraw

I believe it's running RetroArch, as when you load a corrupt file or start without selecting a game it displays the RA menu. If you want to dig deeper you can check out their github or ask in their Discord, the team have been wonderful to work with.