it is running a port of retroarch. i’ve not done much research into it, but i would suggest you do before buying. retroarch hasn’t ported that well to all things.
I ended up purchasing a modded NES Classic with 693 USA and European games, 131 Famicom games, 46 Famicom Disk System games, 87 unlicensed and unreleased NES games, and 68 hacks and homebrew games for NES. It might take a while for me to test out a lot of the games to see if most of them run okay, but if I find any issues, I’ll mention them here. I actually wasn’t planning to buy an NES Classic at all, given that it would only have 30 games, but once I learned that I could get one with just about every NES game ever made, it all of a sudden became more interesting. I think it’s now a great little unit that pays good homage to the original Nintendo.