how to go internal coding of ROW IN FLUTTER code example

Example 1: flutter column widget vertical center

Column(
  mainAxisAlignment: MainAxisAlignment.center,
  crossAxisAlignment: CrossAxisAlignment.center,
  children:children,
)

Example 2: next row column in flutter

var phoneNumber = new Row(
  mainAxisSize: MainAxisSize.min,
  children: [
    Expanded(
      child: new Padding(
        padding: const EdgeInsets.all(20.0),
        child: countryCodePicker,
      ),
    ),
    Expanded(
      child: new Padding(
        padding: const EdgeInsets.all(20.0),
        child: mobileNumber,
      ),
    ),
  ],
);