: Click the download (cloud) icon. The App Store will detect your OS and ask if you want to download the last compatible version for your Mac.
: DMG files are disk images used on macOS for distributing software. If you're looking for a DMG to install GarageBand 10.3.5, you should be cautious about where you download it from. Downloading software from untrusted sources can pose a security risk to your Mac. Garageband 10.3.5 Dmg