This latest update is the 15th for the Global, Exynos, S21 series and is currently being rolled out by Samsung.
This update is for general bugfixes and the August security patch.
Initially, as always, this is a staged roll out to the non-carrier branded, stock Samsung CSC codes first.
The Galaxy S21 Global, Exynos, devices that will receive this update are...
- S21 5G model SM-G991B
- S21+ 5G model SM-G996B
- S21 Ultra 5G model SM-G998B
This latest release replaces the previous G99*BXXU3AUG4 build.
It is a FOTA, (Firmware Over The Air), update and you can check... Settings > Software update... to see if it is available for you. Alternatively, you can connect to the Samsung PC suite, Smart Switch and check via that.
Changelog
- The security of your device has been improved
The update = G99*BXXU3AUGM
Download = 266.69MB
Build date = 29 July 2021
Release date = 04 August 2021
Camera version = 11.0.03.16
One UI version = 3.1
Android Security patch = 01 August 2021
This is the 15th stable update in 192 days. Average = 1 update every 13 days.
(N.B. This post will be updated as more information becomes available)
AUGM System information
Samsung update AUGM
Still 80% left. I'd say that's pretty decent considering 120hz and WQHD 
Previous July update the heat would be much more and would have various red dots to say it got quite warm.
