Samsung is running an Android 10 beta program for the Galaxy S9 and Galaxy Note 9 right now. Since these devices were launched last year, they're getting new features that are a part of Android 10 and One UI 2.0 and some features that were available on this year's flagships even on Android Pie.
Among the latter is a Night mode for the front camera and for the telephoto rear camera, which debuted on the Galaxy Note 10 and later came to the Galaxy S10 and the Galaxy Fold via a software update. On Android 10, Galaxy S9 and Galaxy Note 9 owners can take brighter and more detailed photos with the front camera and the telephoto camera in dark conditions with the dedicated Night mode.
As you can see in the samples below, Night mode makes a noticeable difference. These samples were taken on a Galaxy S10, but you can expect similar results on the Galaxy S9 and Galaxy Note 9 as their cameras aren't different from the Galaxy S10 series. Well, the Galaxy S9 and Note 9 do have a wider aperture for the front camera than the Galaxy S10 and the Galaxy Note 10 series, and that means the end result should be better when you use Night mode to take a selfie.