A 16GB MBP will be the minimum RAM solution. Thus graphics and performance will not be a pleasant experience. VirtualBox support for macOS is still experimental and there are no guest additions available for macOS guests. dmg directly without a need to convert it into. Once you retrieve and install VirtualBox 6.1.12 (or later when available) and its available corresponding extension pack, you will need to follow the included VirtualBox manual for configuring the guest to locate and use that macOS.
Download the Catalina full installer from the Mac App Store, and look for it in the /Applications folder.
