What's related to extension development:
To start,
Fork
git clone https://github.com/[your name]/scratch3-internet.git
git remote add upstream https://github.com/gasolin/scratch3-internet.git
git remote update
npm setup && npm start
http://localhost:8601/
To create a extension
scratch-vm/src/extensions/
scratch3_[name]
scratch-vm/src/extension-support/extension-manager.js
scratch-gui/src/lib/libraries/extensions/index.jsx