[Samsung Galaxy S21 Guides, News] iBowToAndroid:
For those who aren't already aware, G991U is the model number of all of the US carrier versions of the S21. The G991U1 is the model number of the same device, but sold by Samsung SIM unlocked and with no carrier bloatware. Firmware is 100% interchangeable among these models: any U or U1 firmware version can be flashed to any U or U1 device at any time, assuming that you're not trying to downgrade the bootloader. A "U to U" or "U1 to U1" flash does not require a factory reset, but changing from one firmware to the other will require one. Also, U firmware is identical for the 4 base files (AP, BL, CP, CSC) for each individual build, no matter which carrier it comes from. So if you download 2 different carrier versions of the exact same build, the 4 base files will be identical. Where the firmware differs is in the USERDATA file - that's where the carrier bloatware and customizations reside.

Manual Odin flashing is designed for those who:

- want to manually update to a newer version of U firmware on their U device, either because they don't want to wait for their carrier's OTA or because OTAs aren't working on their device, or a different carrier got a new feature/patch before they did

- want to flash U1 firmware to their U device

- want to convert from U1 firmware to carrier U firmware

- want to convert from one carrier's U firmware (with all apps and bloatware) to a different carrier's U firmware (with all apps and bloatware)

For those who aren't already aware, anyone can now download firmware for almost any Samsung device any time that they want, via the various firmware download tools posted here on XDA like SamFirm or Frija. There are threads and info about them here on XDA so I won't waste time explaining - do a search if you're not already familiar with these tools. But they only provide whatever the latest/newest/current version is - nothing older. I pay for a subscription to a site that gets the official releases (usually) before they hit the firmware download tools, so that's where my files come from.

This is not a "beginner's guide to Odin flashing" -type post/thread, so I won't list basic, step-by-step flashing instructions. My threads are geared towards those who already know what they're doing. If you're new to manually flashing Samsung firmware (or haven't done it in a long time), I recommend you consult one of those "beginner's" guides- there are tons of them here on XDA.

Happy flashing!

U firmware: https://www.androidfilehost.com/?w=files&flid=322084

U1 firmware: https://www.androidfilehost.com/?w=files&flid=322085

Notes:

- DON'T QUOTE THE OP WHEN YOU REPLY, kthx

- I have lots of threads just like this one for other Samsung flagships, if it interests you. For those who have already made use of my previous threads for a different device and are upgrading to an S21, welcome back :)

- My zips are compressed extra small for space and bandwidth savings. The default Windows extractor can't handle it, so you'll need a 3rd party tool like 7zip, WinRAR, etc

- it's quite time consuming to download the files, zip them up, and upload them to AFH. And, I do threads like this for every single Sx and Note x model - so I won't be uploading every single version. The only full builds that I upload are ones where I get access to a new bootloader and/or Android version early, before they hit the firmware download tools.

- My naming method for folders on AFH is: build - bootloader - Android version

So "SAT - 1 - 9.0" means the full build number ends in SAT, it's bootloader version 1, and it's Android 9.0. When bootloader and Android versions increment, the first build to have those changes will have its folder notated appropriately.

- HOME CSC file keeps data intact; using the other CSC will wipe data. Flashing USERDATA will also wipe data.

- Since AT&T doesn't allow their firmware to be publicly available (for any of their devices), I'll try to always upload at least one USERDATA per bootloader increment for AT&T (and Cricket, if they get this model). I'll also upload any full builds if AT&T is the only carrier to get that specific build, because you won't be able to find the files anywhere else.

- For identifying USERDATAS, and to know what region/CSC to use in the firmware download tools if you're downloading yourself:

AIO = Cricket (not available via firmware download tools)

ATT = AT&T (not available via firmware download tools)

BST = Boost Mobile

CCT = Xfinity Mobile (Comcast)

CHA = Spectrum Mobile (Charter)

SPR = Sprint

TMB = T-Mobile

TMK = Metro PCS

USC = US Cellular

VZW = Verizon (available via firmware download tools, but Manual Entry is required - Auto will not work)

XAA = U1 firmware

- the correct procedure for switching from one carrier's firmware to another is:

1. Flash full U1 firmware package including non HOME CSC

2. Upon boot, insert a SIM of the carrier whose firmware you want

3. Watch for the popup that says "reboot to apply new carrier settings" etc, and do it

4. After that restart completes, reboot back to Download mode and flash the full U firmware package, with the USERDATA file of the carrier whose firmware you want