iOS simulator showing a bar at the bottom in my horizontally-view game

I made a mobile game where the phone must be held horizontally. It seems that the iOS simulator draws a black line (representing the real-life microphone?) at the bottom of it, even though it should be on the "right wall" if you hold the phone like that.

Is this problem on my end (my code, etc.) or is it just a bug with the iOS simulator? This problem persists on other iPhone models as well. I used Expo and React-Native to make this app.

In fact your assumption that there is a 'problem' is incorrect. This has nothing to do with your code, nor does the line represent the real-life microphone.

The black line you describe is displayed at the bottom of the screen by iOS to users to indicate they can swipe up from there.

This line can be either black or white, depending on what else is being displayed on the screen. The iOS Simulator is accurately simulating what will be seen by users on an actual hardware device.