On some Mac GUIs, such as mine (MacOS High Sierra 10.13.2), you may not be able to view the contents of the .ipa file containing the .app file. Drag the .ipa file to the Applications folder. Then right-click> Show Package Contents. Then you should see the Payload folder and the .app file.
You will need this .app file to open the application in the Xcode simulator.
To do this, only drag the .app file into Applications> Xcode.app (right-click - Show package contents)> Content> Developer> Platforms> iPhoneSimulator.platform> SDK> iPhoneSimulatorXX.Xsdk> Applications
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