Is this useful to anyone else (maybe an OT)? I prepped it for a pal to understand how to get Retroarch (Retail Mode) working on his Xbox. Considering how overwhelmed I was at first, thought it might be helpful to just break it down step by step. I believe this is correct, but feel free to let me know where I goofed?

DOWNLOADING WHAT YOU NEED

- Fill out whitelist application link. You won't be notified when you're whitelisted, so just check the next step every now and then to see if you have.
- When whitelisted, open your Edge Browser on Xbox, go to https://tunip3.github.io/wowee/ and click on the Retroarch link. Download the resulting Restrospective (Retroarch) to your Xbox.
- Download My Files Explorer from Xbox Store ($2.99 US / $3.49 Canadian) to make things easy.
- Open Retroach, click "Online Updater" and one by one, do all of the updates. This'll take around 5-10 minutes.
- If you have additional Bios (PCSX2 for Playstation 2 Games, for example) add to USB drive.
- Put ROMS you own on exeternal USB hard drive/thumbstick

PREPPING MY FILES EXPLORER & USB HARD DRIVE

Open My Files Explorer:
Click three dots next to "This Device" (can be hard to spot - basically, three little dots near the address bar)
Click Add Folder To Library
Click Xbox console symbol on the left hand side, then back/menu button to highlight "Select" - Press A to confirm.
Click "Isolated Storage" folder
Below the address bar, click "Packages"
Click 1e4... (dev mode) or 52269xbonedev1.Retrospective (retail mode) for Retroarch
Click "Local State" folder
Click System + paste in the folder of any Bios you want to add (the full folder). If you aren't pasting in additional Bios at this point, no problem, you can do it later by navigating here.
For External/USB Hard Drive Transfer:
Insert USB into XBOX
Open My Files Explorer
Click three dots next to "This Device" (again, the three little dots near the address bar)
Click Add Folder To Library
Click the External Hard drive symbol on the left then press the back/menu button to "Select" - Press A
Click E: (might be D for some people)
All of your owned ROMS you added to the USB will show up for you
Now you're all prepped.

ADDING ROMS (once the above is set-up, this is all you'll have to do each time you want to add a ROM)
The ROMS should be clearly named and added to your USB hard drive. Recommended file types: .pbp for PS1, .wbfs for Wii, .z64 for Nintendo 64, and .smc or .sfc for SNES. Most files should work, but PS1 and Wii gave me trouble until I focused on those file types.
Open My Files Explorer. Click Removable storage. Hover over the folder or the ROMS you want to add. Press start and click "copy."
Click "Isolated Storage" > Packages (listed underneath address bar) > the either the .1e4 (dev mode) or 52269xbonedev1.Retrospective (retail mode) folders, depending what you see > then click Local State
Paste your ROMS here by pressing "Start" and "Paste." I'd recommend making a folder either "ROMS" or broken down by platform ("SNES" / "WII" etc.), but also, it doesn't matter beyond being organized. As long as they are pasted here, they work.
At this point, you're good to play.

PLAYING ROMS ONCE THEY ARE ADDED:
Basic way this works is click "Load Content" then select the appropriate drive for your ROMS. This would be U: for internal storage (most likely what you're doing if you folllowed the above steps), but you can also play directly off of the USB D or E: if that's where they are.
Click the rom, then you'll be given a choice of different emulators. Most work great, but see the note about PS1/Wii game file types above.
That's it!?
RETROARCH TIPS (once you get games working, I suggest doing these quality of life improvements):
Go into Retroarch settings. Somewhere in User Interface or Menu Settings, there should be an option to edit your "hot keys." Make sure to add one to toggle the Retroarch menu with whichever input you prefer (I suggest either holding select for 2 seconds or L3 + R3 since they won't conflict with in game controls). This will let you change Roms on the fly or quit out of one instead of hard closing the app. It also lets you mess around with other settings once you're in the game.
If you are running a game and it looks/plays great, I recommend opening the menu (whichever toggle you did above) and clicking "Add To Favourites." This makes it so that rather than go through the 2-3 steps of loading content/selecting core each time, and trying to remember which cores/emulators work best, you can just navigate to your Favorites list and click the name of the game and be playing in seconds. It will remember the settings you used. Also, it's just kind of badass to have one big list of games from SNES to N64 to PS1 to Wii and have them all launchable in a click.
SUGGESTED EMULATORS/CORES TO CHOOSE WHEN GIVEN THE CHOICE (this is what I've found - I'm sure the experts may have other ideas)?
NES: Nestopia
SNES: bsnes-mercury-balanced
N64: Mupen64 Plus
Gameboy: Gambatte
GBA: mGBA
PSX: Mednafen or PCSX Rearmed
Genesis: Genesis Plus GX
32X: PicoDrive