Structure of Scratch
Unlike the previous versions of Scratch, Scratch 3.0 is separated into multiple packages, each of which is responsible for part of Scratch. For most mods, you probably do not need to modify (or even have a modifiable copy of) all the packages.
The Important badge is used to indicate components that are important in modding Scratch. Most of the components provide important functionality that Scratch would not work without or have a degraded experience without.
scratch-gui Important
scratch-gui is the main package of Scratch. It brings together all the other packages and contains most of the user interface of Scratch.
scratch-vm Important
scratch-vm is responsible for executing the project code. This package contains the code that is executed when a block runs. It also contains the code for the built in extensions.
scratch-blocks Important
scratch-blocks is the actual code editor part of the Scratch editor. It provides the block toolbox, workspace and the visuals of the blocks themselves. scratch-blocks is a fork of Google's Blockly library.
scratch-render
scratch-render contains the code responsible for rendering the project stage.
scratch-paint
scratch-paint is the Scratch paint editor.
scratch-audio
scratch-audio handles playing of audio in the project.
scratch-desktop
scratch-desktop is the Electron-based desktop application for Scratch.
scratch-render-fonts
scratch-render-fonts contains the fonts available for use in Scratch.
scratch-storage
scratch-storage handles storage of Scratch projects.
scratch-svg-renderer
scratch-svg-renderer renders SVGs, while maintaining compatibility with Scratch 2.0's SVGs.
scratch-sb1-converter
scratch-sb1-converter converts old Scratch 1.4 (.sb) files into Scratch 2.0 (.sb2) files, which can then be converted to Scratch 3.0 (.sb3) files.