Xcode simulator/device layout problem

Hello everyone.

I recently made my own app and released it to app store. Before I released it, I use my device(IPhone 7), and use various simulator on xcode to find out there are no layout problems. When I run my app on IPhone 7 and simulator(IPhone 11 pro, IPhone 12...), it worked great. But when real customer downloaded my app through app store, they found out there are layout problem, such as no text on button, white screen things.(They use IPhone 12 or IPhone 11)

The problem is that I only have Iphone 7, and when I tested it through my device, there was no problem at all. And simulator also didn't show any problem on layout.
So how can I test my app to assure that there are no layout problem? Do I have to buy another device like IPhone 12?
Why there is difference between simulator and device on UI layout?

Thanks for reading.
Xcode simulator/device layout problem
 
 
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