Samsung has started rolling out One UI 4 to the Galaxy S21, Galaxy S21 Plus and Galaxy S21 Ultra globally. The OS update, based on Android 12, will eventually be available on older Galaxy S and Galaxy Note series handsets, before making its way to the Galaxy A and Galaxy Tab S series.
https://www.notebookcheck.net/Samsung-starts-One-UI-4-and-Android-12-upgrades-with-the-Galaxy-S21-Galaxy-S21-Plus-and-the-Galaxy-S22-Ultra.579272.0.html