Official Dart website https://dart.dev/
Official Flutter website https://flutter.dev/
Official Package manager for Dart and Flutter https://pub.dev/
Flutter Gems website groups the packages and plugins published in pub.dev by category. You can easily get the list of packages and plugins available for your need. https://fluttergems.dev/
Blogs
1). Official Dart Blog https://medium.com/dartlang
2). Official Flutter Blog https://medium.com/flutter
3). Official Firebase Blog https://firebase.blog/category/flutter
4). Google Dev Library https://devlibrary.withgoogle.com/products/flutter
5). Flutter Community https://medium.com/flutter-community
6). Very Good Ventures https://verygood.ventures/blog-categories/flutter
7). Codemagic https://blog.codemagic.io/tags/flutter/
8). Stream https://getstream.io/blog/topic/engineering/flutter/
9). Code With Andrea https://codewithandrea.com/
10). Vandad Nahavandipoor https://vandad.sh/category/framework/flutter/
YouTube Channels
1). Flutter official Channel https://www.youtube.com/c/flutterdev
2). Flutter Community https://www.youtube.com/c/FlutterCommunityVideos
3). The Flutter Way https://www.youtube.com/c/TheFlutterWay
4). Flutterly https://www.youtube.com/c/Flutterly
5). Vandad Nahavandipoor https://www.youtube.com/c/VandadNP
6). FilledStacks https://www.youtube.com/c/FilledStacks
7). Flutter Explained https://www.youtube.com/c/FlutterExplained
8). Johannes Milke https://www.youtube.com/c/JohannesMilke
9). Majid Hajian https://www.youtube.com/c/mhadaily/featured
10). Robert Brunhage https://www.youtube.com/c/RobertBrunhage
11). Reso Coder https://www.youtube.com/c/ResoCoder
12). Fun with Flutter https://www.youtube.com/c/FunwithFlutter/featured
13). RetroPortal Studio https://www.youtube.com/c/RetroPortalStudio/featured
14). Tadas Petra https://www.youtube.com/c/TadasPetra/featured
15). Flutter Mapp https://www.youtube.com/c/FlutterMapp
16). diegoveloper https://www.youtube.com/user/diegoveloper