Apple delivers the macOS installer and applications through the app store since the launch of OS X Lion. The macOS installer has a .app extension when you get it through the official channel. Unlike the macOS DMG installer, which Apple doesn’t provide nowadays, the app version of the macOS install package is not bootable. Hence it is not mountable nor compatible with operating systems like Windows. For some reason, if you want macOS in .DMG or .ISO format, there is a method to convert the .app file. Unfortunately you need a macOS system to convert .app to .dmg to .iso image.
Snow leopard 10.6.0 dmg.
Snow leopard 10.6.0 dmg.