Correct, its setup per your global settings, so it already knows what buttons you have, so you just swap them around if needed. Personally the only core i even change is N64 everything else i leave as is, but each to there own.
You use the same button to close it as you use to open it, no more difficult than that, you should be saving your remap before you close retroarch though.
Just set your controls here.
Then backup one level to here and save.