The Retroarch experience is amazingly convenient once you learn how to program everything and get it all set up. For starters, make sure you stuck all of the core .nro files into the retroarch/cores/switch directory, as that's the default place that Retroarch will look for them. As for the roms, I sort mine into its own folder located in the root of the SD card, in a folder called Roms. I also sort all of my roms by type in their own folders within the Roms directory. For example, my GBA roms are in a folder called GBA. Genesis are in a folder called Genesis. Etc etc.
Once you have this done, launch RetroNX and head into the settings. Scroll down to the bottom and select the option called "Directory". In this menu, scroll down to where it says "Core" and select that option. Then, browse to the cores folder located at retroarch/cores/switch and set this directory as the default directory.
At this point, you should be able to go back to the main menu and select a core. Select "Load Core" and it should automatically show every core you have loaded into the cores folder. Select the core you wish to load and it will load the core and be ready to play. Each core is a specific emulator, so for example, if you load vba-next.nro, you will select the core that plays GBA roms. After the core is loaded, you should then be able to browse to your directory that contains your .gba files and load whichever rom you wish. If you want to load roms for a different emulator, you have to first switch to the corresponding core to do so.
I hope this little explanation helps!
