@Aryan this one in particular uses a special type of chip that has the ability to emulate the exact hardware in which the ROM was made to run. Most emulators are software emulations that use Linux to run the emulator proggies, which makes them slow and many times, they don't emulate the game very well
I remember seeing it being released a while ago, but they said it'd still take a couple years to make it really interesting. These days, it's looking really good. I saw a video of the games running, they look identical to the originals, really beautiful