These days our phones have huge screens with no bezels, and most of them have on-screen touch controls. And while buttons on the screen are nice, they waste space. As a result, Apple, OnePlus, Google and now Samsung have introduced "gesture controls".
Gesture Controls allow you to enjoy the entire 5.8 or 6.2-inch screen of your Galaxy S9/S9+ without buttons wasting space as the bottom. Instead, you'll have to learn a few swipes and gesture controls, but it's really easy to master.
We've seen Apple's iPhone X adopt a similar control system, and Samsung just added gestures to the Galaxy S9 with the recent Android 9 Pie One UI update in early 2019. Try the gesture controls and feel like you're controlling a phone from the future, like Minority Report.