Samsung has had major issues providing quality the Android 12-based One UI 4 update and if you don’t know they even started and then had to halt the rollout a few times already.
The One UI 4 update for the Galaxy S21, Z Flip 3 and Z Fold 3 had some Google Play systems compatibility issues according to Samsung. They had to fix that as soon as possible. So after teaming up with Google specifically to have these problems solved, Samsung is now rolling out One UI 4 again to the Galaxy S21, Z Flip 3, and Z Fold 3, thanks to XDA-Developers for these infos.
The update is now rolling out globally, so if you own a Galaxy S21 series phone or Galaxy Z Flip 3 and Z Fold 3 foldable devices, you should be getting One UI 4 any moment. Apart from the Android 12-based One UI 4 improvements, the update also includes the December 2021 security update.