Builds iOS app (only once) for multiple simulators.

boot_app_multiple_simulators.rb

DEFAULT_SIMULATORS = ['iPhone 5s', 'iPhone 8', 'iPhone X', 'iPhone Xʀ']
PROJECT_PATH = # Project path.
SCHEME = ... # (Your scheme)
CONFIGURATION = ... # (usually Debug or Release)
APP_ID = # Your app id. i.e: com.company.app
APP_NAME = # The output name of your app.

platform :ios do

  desc "Build the app"
  lane :build do
    simulator_sdk_path = sh("xcrun --show-sdk-path --sdk iphonesimulator").to_s.strip()
    sh("xcodebuild -configuration #{CONFIGURATION} -scheme #{SCHEME} -project #{PROJECT_PATH} -sdk #{simulator_sdk_path} build -UseModernBuildSystem=NO")
  end

  # Before running this lane, ensure that you built the app in Xcode first.
  # Example specifying simulators: fastlane run_multiple_simulators simulators:"iPhone 5s, iPhone 8"
  desc 'Run App in multiple simulators'
  lane :run_multiple_simulators do |options|
    simulators = options[:simulators].nil? ? DEFAULT_SIMULATORS : options[:simulators].split(',').map { |device| device.strip }
    derivedDataQuery = sh "xcodebuild -project #{PROJECT_PATH} -scheme #{SCHEME} -showBuildSettings | grep -m 1 \'BUILD_DIR\' | grep -oEi \"\/.*\" | tail -1"
    derived_data_path = derivedDataQuery.split.last + "/Debug-iphonesimulator/#{APP_NAME}/"

    if !File.exist?(derived_data_path)
      UI.error("#{derived_data_path} doesn\'t exist! Building app...")
      build
    end

    # Launch simulator app
    sh('open /Applications/Xcode.app/Contents/Developer/Applications/Simulator.app/')

    # First boot all simulators because it takes some seconds
    simulators.each do |simulator|
      begin
        sh "xcrun simctl list | grep Booted | grep -w \'#{simulator}\' "
      rescue
        sh "xcrun simctl boot \'#{simulator}\'"
      end
    end

    # Install and launch the app
    simulators.each do |simulator|
      sh("xcrun simctl install \'#{simulator}\' #{derived_data_path}")
      sh("xcrun simctl launch \'#{simulator}\' #{APP_ID}")
    end

end
end

Note: If you want to use a workspace, use
-workspace workspace-path instead of -project project-path