Wondering if anyone can give insight on what's up here. I installed Retroarch through Steam as a frontend for emulators. However, the "Online Updater" button you can use to download emulators isn't in the Steam version, only the version you download from the website; on the Steam version, you have to use the emulators the Retroarch devs provide through Steam. Their statement when someone asked about the lack of the "Online Updater" button:
"The cores are distributed as DLC, in order to stay within Steam's infrastructure (which was required for us to be on the store). All of the cores provided were given approval by the authors of the core code (with more on the way).
Right click RetroArch in your library, go to properties, go to the DLC tab and check all the boxes you wish to download. If RetroArch was open, restart it.
The rest of the assets, aside from thumbnails, (currently not provided) are automatically included within the main RetroArch install."