brew cask is a whole separate quasi-package management system built around pre-built macOS GUI applications distributed/installed as .app folders, same as you'd

brew cask is a whole separate quasi-package management system built around pre-built macOS GUI applications distributed/installed as .app folders, same as you'd get downloading a .dmg from the web them doing a drag and drop install. Think of it as an installer fetcher and runner, as opposed to a collection of integrated and uniformly built packages. It's great, because downloading and running installers manually by clicking around sucks. But it's totally separate from the rest of Homebrew, which is a monorepo of build recipes targeting a separate base system, similar in that basic way to things like pkgsrc and FreeBSD Ports.