How can I launch the iOS Simulator from Terminal?

Just use

 open -a simulator

For some specific device

1. open terminal
2. xcrun simctl list
3. get udid of the device you want to launch
4. Paste this in the terminal
5. open -a Simulator --args -CurrentDeviceUDID 0566AC33-9B91-2DR2-B5BB-C916D3BA8MD3

In case of react-native just run npx react-native run-ios It will automatically open the simulator. Use above method if you want to run some specific device.

How to close simulator, rotate or to take screenshot.

1) Simple way to close single simulator
-> Select required Simulator
-> Click Command + W (⌘W).

2) If you want to minimise simulator
-> Select required simulator
-> Click Command + M (⌘M).

3) For screen shot of simulator
–> Select required simulator
–>Select Commend + S (⌘S).

4) If you want to rotate simulator(for landscape mode)
-> Select required simulator
-> Click Command + left arrow or right arrow.

Or simply use the menu at top when simulator is selected.

React Native Change Default iOS simulator

See below examples

react-native run-ios --simulator="iPhone 6 Plus"
react-native run-ios --simulator="iPhone 6s"
react-native run-ios --simulator="iPhone 6s Plus"
react-native run-ios --simulator="iPhone 7"
react-native run-ios --simulator="iPhone 7 Plus"
react-native run-ios --simulator="iPhone 8"

If two devices with different iOS version exists then rename those as follows.

Xcode -> Window -> Devices and Simulators -> Simulators.