The toolbox is the place where users get blocks. Usually it is displayed on one side of the workspace.
A flyout toolbox has a single set of blocks that are displayed at all times. A category toolbox has multiple sets of blocks that are arranged into different categories.
Toolboxes can be defined using JSON (as of the September 2020 release) or XML. The JSON format is preferred.
This section mainly focuses on how to specify the structure of your toolbox (i.e. what categories it has, and what blocks they contain). If you want more details about how to change the appearance of your toolbox, see Category appearance, the Customizing a Blockly toolbox codelab, and the 2021 Toolbox APIs talk.