Just launch Retroarch itself without a game and you are in the user interface. You can maneuver over to inputs section and see what the stock mappings are. From there you can set your custom controls which would affect all cores or you can load a core and set custom controls for each core.
The benefit of RA is running a bunch of systems with only one emulator, bezels, save states to name a few. Plus being able to set custom controls per core, per game, even per folder. There are a ton of tutorials on the individual emulators on how to set them up. The ones LB puts out are just that, tutorials about getting LB set-up.