21.02.27 기준으로
TextSpan사용 예제 코드도 넣어놨습니다 :)
https://velog.io/@adbr/flutter-TextSpan-example-apply-font-style#2-used-textspan-code

1. 폰트가져오기(다운로드)

Google Fonts
https://fonts.google.com/

눈누 (한글 상업용으로 사용가능한 글꼴 많음)
https://noonnu.cc/index

2. 폰트 ttf 파일 프로젝트에 넣어주기

 project
     ㄴ android
     ㄴ assets
    	ㄴ font
    	  ㄴ {파일 넣는 곳}
        ㄴ ...
     ㄴ build
     ㄴ ios
     ㄴ lib
     ㄴ ...

3. pubspec.yaml 수정

(pubspec.yaml 60라인쪽에있음, 원래 주석되어있던 부분을 품)

  fonts:
    - family: Noto_Serif_KR
      fonts:
        - asset: assets/font/Noto_Serif_KR/NotoSerifKR-ExtraLight.otf
          weight: 200
        - asset: assets/font/Noto_Serif_KR/NotoSerifKR-Light.otf
          weight: 300
        - asset: assets/font/Noto_Serif_KR/NotoSerifKR-Regular.otf
          weight: 400
        - asset: assets/font/Noto_Serif_KR/NotoSerifKR-Medium.otf
          weight: 500
        - asset: assets/font/Noto_Serif_KR/NotoSerifKR-SemiBold.otf
          weight: 600
        - asset: assets/font/Noto_Serif_KR/NotoSerifKR-Bold.otf
          weight: 700
        - asset: assets/font/Noto_Serif_KR/NotoSerifKR-Black.otf
          weight: 900

4. 적용

1) 전체적용

import 'package:flutter/material.dart';
import 'package:lenah_basic/home.dart';

void main() => runApp(MyApp());

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(
        fontFamily: 'Noto_Serif_KR',
        brightness: Brightness.light,
        backgroundColor: Colors.white,
        accentColorBrightness: Brightness.light,
        visualDensity: VisualDensity.adaptivePlatformDensity,
        primaryColor: Colors.blueGrey,
        primarySwatch: Colors.blueGrey,
        scaffoldBackgroundColor: Colors.white,
      ),
      home: HomePage(),
    );
  }
}

2) 특정 텍스트 적용

import 'package:flutter/material.dart';
    
class HomePage extends StatelessWidget {
  const HomePage({Key key}) : super(key: key);

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text(
          'test 테스트1',
          style: TextStyle(fontFamily: 'NotoSans'),
        ),
      ),
    );
  }
}

3) bold 적용하고싶다면

pubspec.yaml에서 weight 700 일 때 위 폰트를 따르겠다고 지정해주었기 때문!

style: TextStyle(fontFamily: 'NotoSans', fontWeight: FontWeight.w700),

4) 기기 폰트 사이즈에 의존하고 싶지않다면?

MaterialApp에 builder 속성 추가

builder: (context, child) => MediaQuery(
  child: child,
  data: MediaQuery.of(context).copyWith(textScaleFactor: 1.0),
),

전체 코드

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(
        fontFamily: 'Noto_Serif_KR',
        brightness: Brightness.light,
        backgroundColor: Colors.white,
        accentColorBrightness: Brightness.light,
        visualDensity: VisualDensity.adaptivePlatformDensity,
        primaryColor: Colors.blueGrey,
        primarySwatch: Colors.blueGrey,
        scaffoldBackgroundColor: Colors.white,
      ),
      home: HomePage(),
			builder: (context, child) => MediaQuery(
        child: child,
        data: MediaQuery.of(context).copyWith(textScaleFactor: 1.0),
      ),
    );
  }
}

cf. blod weight

From the docs we get this list of constants:

w100 Thin, the least thick
w200 Extra-light
w300 Light
w400 Normal / regular / plain
w500 Medium
w600 Semi-bold
w700 Bold
w800 Extra-bold
w900 Black, the most thick

cf. https://stackoverflow.com/questions/53687104/flutter-not-picking-custom-fonts-based-on-fontweight

결과 화면

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