To turn that app into a DMG for virtual machines or storage, you can use the Terminal: Open .
The perceived "exclusivity" of the macOS Big Sur DMG download is a result of architectural changes in macOS installation files, specifically the move toward chunked, integrity-checked packages. While direct DMG downloads are not provided by Apple, the tools to create them are built into macOS.
Running Big Sur on Parallels, VMware, or VirtualBox.
