Anyone have any idea how to add dreamcast, genesis, pokemon mini, or any of the atari systems to show up? (default os)

I know retroarch supports these systems? I've also had previous miyoo devices that at least had pokemon mini, atari 2600, 5200, and 7800 on it? I noticed someone else had a similar question but no one had replied, figured id ask as well with the other systems ive had problems with. I can even see on retroarch the cores for it, but i just cant seem to find a way to push them onto the original operating system? Any ideas?