As an installer program, like on Windows. Or as an installable package (Mac OS should have an integrated package management system like Homebrew).
I don't know the exact purpose behind the "mount a virtual drive to install an app" model, but it seems far too deliberate to not have a purpose.
For me, AppCleaner is one of the first things I download on a new machine.
% brew uninstall <appname>
Or uninstall and remove all of its cruft as well via: % brew zap <appname>