Building with Xcode command line tools

To build an app with Xcode command line tools xcrun xcodebuild should be used. Here is an example on how to build an app called MyApp for iOS simulator target:
xcodebuild -project MyApp.xcodeproj \
-scheme MyApp \
-configuration Debug \
-destination 'generic/platform=iOS Simulator' \
The .app file will then be located build/ folder, which can be modified via the last parameter of the script.

Building with Fastlane

If you use fastlane for your automation pipelines the script should look the following way:
configuration: build_config[:configuration],
scheme: build_config[:scheme],
workspace: build_config[:xcode_workspace],
xcargs: "-quiet -sdk 'iphonesimulator' -destination 'generic/platform=iOS Simulator'",
derivedDataPath: IOS_DERIVED_DATA_PATH # this will contain the .app which we need later on

Building with Flutter

If you use Flutter to build your app you can create a debug build for simulators using the following command:
flutter build ios --debug --simulator
You can then find your app file in the build/ios/iphonesimulator/ directory.