Do not download "GarageBand 10.4 DMG" from third-party file-hosting sites. The risk of malware is non-trivial. Instead, use the App Store's "Compatible Version" feature to download the legitimate build directly to your Apple ID. If you are an IT administrator looking to deploy 10.4, utilize Apple Business Manager or download the specific .pkg installer via terminal command or software update catalog tools.

A is a Apple Disk Image file. It acts as a virtual folder that contains the installation files for macOS applications.