CrossAxisAlignment 질문

import 'package:flutter/material.dart';

class HomeScreen extends StatefulWidget {
  const HomeScreen({super.key});

  @override
  State<HomeScreen> createState() => _HomeScreenState();
}

class _HomeScreenState extends State<HomeScreen> {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: SafeArea(
        child: Column(
          crossAxisAlignment: CrossAxisAlignment.start,
          children: [
            Row(
              mainAxisAlignment: MainAxisAlignment.spaceBetween,
              children: [
                Text('랜덤숫자 생성기'),
                IconButton(
                  onPressed: () {},
                  icon: Icon(
                    Icons.settings,
                  ),
                ),
              ],
            ),
            Expanded(
              child: Column(
                mainAxisAlignment: MainAxisAlignment.center,
                children: [
                  Text('123'),
                  Text('456'),
                  Text('789'),
                ],
              ),
            ),
            Container(
              width: double.infinity,
              child: ElevatedButton(
                onPressed: () {},
                child: Text('생성하기!'),
              ),
            ),
          ],
        ),
      ),
    );
  }
}

그냥 수업 내용인데, 왜 첫번째 Column에서는 container나 sizedBox하지 않고 그냥 crossAxisAlignment가 바로 적용되는 건가요?? 혹시 SafeArea 영역이 width가 최대라서 바로 횡축 정렬이 가능해지나요?