It's not a system app, it's a system feature in Android 9: as an alternative to auto rotation you get a rotate button on the nav bar for a short while after rotating the phone. I've been using it for almost 10 months now and love it, since I've never found auto rotate to work right for me on any device.

However, it sounds like there's something funny on your phone: it shouldn't be sitting on top of the carriage return key. I've never seen that, and as noted I've been using Android 9 since last Summer. It might be that Samsung decided that even with the nav bar hidden you would want this button to appear, unlike its behaviour on other devices where if the nav bar is hidden the button is hidden too (I don't see the nav bar in your screenshot so assume you have it hidden). It might be a bug in their ROM. Or it might be that there's something odd about your setup, or a glitch following the update. If other S9 owners on Android 9 respond we'll know whether it's a glitch in your phone or a general feature of the S9's Android 9 ROM.

As a workaround you could try a navigation bar customisation app such as Custom Nav Bar and see whether you can remove it that way. Or maybe toggle some of the nav bar settings like the auto hide feature, just in case something's got confused, or try clearing the system cache from recovery (if the S9 still has that option - it's tending to be removed in recent phones). But those are just semi-random "try it in case" guesses. If it's a feature of the S9 and third party solutions don't fix it (or cause other problems) then all I can think of is to tell Samsung that this was not a helpful tweak to make.