There are two different types of OS one is Mac OS X Lion 10.7 ISO, DMG and the other one is OS X mountain lion. If you look for apps outside the store, you’ll find different kinds of installers: DMG files with apps in them, PKG installers, and simple applications inside ZIP archives. Sure, there’s the Mac App Store, but not everything is in there.
If you’re switching to macOS from Windows, you might be confused about installing software. Install macOS 10.15 Catalina on VirtualbOx. Here is the guide to install macOS Catalina on VirtualBox on windows. For that, you have to download the macOS Catalina VMDK file. If you are on a mac, simply install on the VirtualBox, if not, it means you are using a Windows OS, the dmg file won’t help at all.
