Astra is the number one most used WordPress theme. Astra Pro is a premium add-on for the Astra theme which enhances the capabilities of the free Astra theme by providing additional features and options. The Astra Pro Site Builder is a module of Astra Pro that lets you create custom headers, footers, single and archive templates, as well as other theme templates.
Astra is a general purpose Classic theme that includes support for Elementor, Beaver Builder, and other page builders, including the Gutenberg Editor. The Site Builder uses Gutenberg blocks from Spectra pro for displaying content elements. It uses WordPress hooks to control placement locations.