https://wccftech.com/galaxy-s9-and-galaxy-note-9-can-now-run-one-ui-4-1/

It is safe to say that at the moment, Samsung is one of the most generous companies when you are talking about the latest updates. The company has released the One UI 4.1 updates on several phones like the Galaxy Note 10 Lite and the rollout is currently happening on other phones, as well. However, you can actually go ahead and install the One UI 4.1 update on the Galaxy S9 and Galaxy Note 9, as well, and considering how the device did not receive even Android 11, this is a great way to update your phone if you want to.

The ROM is developed by AlexisXDa who managed to port One UI 4.1-based Android 12 firmware to both the Galaxy Note 9 and Galaxy S9.