The brand.twig view will display the individual details of a brand and the products that it contains.
In most circumstances the brands page will extend the collection.twig template because it's format will follow the usual display of collection pages on your theme.
The brand page has access to the brand object so displays details about the current brand being viewed such as the title/name, description and image.
The brand page has access to the project object to display products assigned to the brand. These will usually be displayed using the items partial.