中介绍了MaterialApp的默认字体
/flutter/packages/flutter/lib/src/material/typography.dart
在iOS上，默认文本主题为

static const TextTheme whiteCupertino = TextTheme(
    display4   : TextStyle(debugLabel: 'whiteCupertino display4',   fontFamily: '.SF UI Display', inherit: true, color: Colors.white70, decoration: TextDecoration.none),
    display3   : TextStyle(debugLabel: 'whiteCupertino display3',   fontFamily: '.SF UI Display', inherit: true, color: Colors.white70, decoration: TextDecoration.none),
    display2   : TextStyle(debugLabel: 'whiteCupertino display2',   fontFamily: '.SF UI Display', inherit: true, color: Colors.white70, decoration: TextDecoration.none),
    display1   : TextStyle(debugLabel: 'whiteCupertino display1',   fontFamily: '.SF UI Display', inherit: true, color: Colors.white70, decoration: TextDecoration.none),
    headline   : TextStyle(debugLabel: 'whiteCupertino headline',   fontFamily: '.SF UI Display', inherit: true, color: Colors.white,   decoration: TextDecoration.none),
    title      : TextStyle(debugLabel: 'whiteCupertino title',      fontFamily: '.SF UI Display', inherit: true, color: Colors.white,   decoration: TextDecoration.none),
    subhead    : TextStyle(debugLabel: 'whiteCupertino subhead',    fontFamily: '.SF UI Text',    inherit: true, color: Colors.white,   decoration: TextDecoration.none),
    body2      : TextStyle(debugLabel: 'whiteCupertino body2',      fontFamily: '.SF UI Text',    inherit: true, color: Colors.white,   decoration: TextDecoration.none),
    body1      : TextStyle(debugLabel: 'whiteCupertino body1',      fontFamily: '.SF UI Text',    inherit: true, color: Colors.white,   decoration: TextDecoration.none),
    caption    : TextStyle(debugLabel: 'whiteCupertino caption',    fontFamily: '.SF UI Text',    inherit: true, color: Colors.white70, decoration: TextDecoration.none),
    button     : TextStyle(debugLabel: 'whiteCupertino button',     fontFamily: '.SF UI Text',    inherit: true, color: Colors.white,   decoration: TextDecoration.none),
    subtitle   : TextStyle(debugLabel: 'whiteCupertino subtitle',   fontFamily: '.SF UI Text',    inherit: true, color: Colors.white,   decoration: TextDecoration.none),
    overline   : TextStyle(debugLabel: 'whiteCupertino overline',   fontFamily: '.SF UI Text',    inherit: true, color: Colors.white,   decoration: TextDecoration.none),
  );

在Android上，默认文本主题为

static const TextTheme whiteMountainView = TextTheme(
    display4   : TextStyle(debugLabel: 'whiteMountainView display4',   fontFamily: 'Roboto', inherit: true, color: Colors.white70, decoration: TextDecoration.none),
    display3   : TextStyle(debugLabel: 'whiteMountainView display3',   fontFamily: 'Roboto', inherit: true, color: Colors.white70, decoration: TextDecoration.none),
    display2   : TextStyle(debugLabel: 'whiteMountainView display2',   fontFamily: 'Roboto', inherit: true, color: Colors.white70, decoration: TextDecoration.none),
    display1   : TextStyle(debugLabel: 'whiteMountainView display1',   fontFamily: 'Roboto', inherit: true, color: Colors.white70, decoration: TextDecoration.none),
    headline   : TextStyle(debugLabel: 'whiteMountainView headline',   fontFamily: 'Roboto', inherit: true, color: Colors.white,   decoration: TextDecoration.none),
    title      : TextStyle(debugLabel: 'whiteMountainView title',      fontFamily: 'Roboto', inherit: true, color: Colors.white,   decoration: TextDecoration.none),
    subhead    : TextStyle(debugLabel: 'whiteMountainView subhead',    fontFamily: 'Roboto', inherit: true, color: Colors.white,   decoration: TextDecoration.none),
    body2      : TextStyle(debugLabel: 'whiteMountainView body2',      fontFamily: 'Roboto', inherit: true, color: Colors.white,   decoration: TextDecoration.none),
    body1      : TextStyle(debugLabel: 'whiteMountainView body1',      fontFamily: 'Roboto', inherit: true, color: Colors.white,   decoration: TextDecoration.none),
    caption    : TextStyle(debugLabel: 'whiteMountainView caption',    fontFamily: 'Roboto', inherit: true, color: Colors.white70, decoration: TextDecoration.none),
    button     : TextStyle(debugLabel: 'whiteMountainView button',     fontFamily: 'Roboto', inherit: true, color: Colors.white,   decoration: TextDecoration.none),
    subtitle   : TextStyle(debugLabel: 'whiteMountainView subtitle',   fontFamily: 'Roboto', inherit: true, color: Colors.white,   decoration: TextDecoration.none),
    overline   : TextStyle(debugLabel: 'whiteMountainView overline',   fontFamily: 'Roboto', inherit: true, color: Colors.white,   decoration: TextDecoration.none),
  );

并且可以通过以下代码检索字体系列

DefaultTextStyle.of(context).style.fontFamily

MaterialApp的默认字体为roboto，这是一种Google字体。
https://fonts.google.com/specimen/Roboto

默认字体取决于操作系统：

*Android使用Roboto字体。
*iOS使用弗朗西斯科字体（特别是SF Pro Display）。

Flutter应用项目的默认字体系列为
Roboto -获取它在这里：https://fonts.google.com/specimen/Roboto#standard-styles
默认情况下显示在Andriod OS上。
对于iOS设备，
弗朗西斯科字体-在这里获取：https://developer.apple.com/fonts/
要在flutter项目中默认使用其中任何一个（也取决于您的主题设置），您可以导入其中一个或两个：
安德里欧-import package:flutter/material.dart。
适用于iOS -import package:flutter/cupertino.dart。
你也可以使用其它字体（添加它的文件夹到根文件夹或创建资源文件夹）并在pubspec.yaml中包含字体定义

要获取当前字体系列：
DefaultTextStyle.of(context).style.fontFamily
默认字体系列将基于设备：https://api.flutter.dev/flutter/material/Typography/Typography.material2021.html

机器人适用于安卓系统. https://api.flutter.dev/flutter/material/Typography/blackMountainView-constant.html
弗朗西斯科适用于iOS. https://api.flutter.dev/flutter/material/Typography/blackCupertino-constant.html