It runs android and comes pretty bare. You have to install your own standalone emulators, set up retroarch, and supply all your own roms. I'm a bit of a tech dork so setting it up was part of the fun for me but if you want it ready to go out of the box this is not the device for you.
I use Daijisho as a front end to browse through all the games and platforms and its been working well for me. I followed the Retro Game Corps video on setting it all up, that guy knows his stuff.
I've been thinking about grabbing a steam deck for a while and I think a lot of those little handhelds would complement it pretty nicely as they're actually pocketable, though your DS might fill that niche for you already. Steam deck for me would be just be chilling on the couch playing games but the rp3+ seems like it's a lot easier to just throw in a bag and go.