I need to test my app on iPhone 4S.
In Xcode 10, I've went to Xcode > Preferences > Components > Simulators
and I've downloaded "iOS 9.3 Simulator"
But I don't see it on the simulator list next to the Run button.
Any idea, how to run this iOS 9.3 simulator in Xcode 10?
You have to create a simulator using the simulator os version. You have only downloaded the iOS version software.
To add a device follow: Window > Devices & Simulators. (or cmd + shift + 2)
Then at the bottom of the list press the + icon. This should allow you to add a device running the selected iOS version.