Note: The installation of High Sierra on my Mac consumed about 29 GB of space.
Once booted from the High Sierra flash drive, you can install High Sierra in a new APFS volume. See the Apple website How to create a bootable installer for macOS for instructions. To install High Sierra, you will have to copy the installation files to a flash drive. You can click on Quit to exit the installer. After High Sierra finishes downloading, you will get the following popup message. You can download High Sierra by accessing the link on the Apple website How to upgrade to macOS High Sierra. You do not have to downgrade to a earlier version of MacOS, you just need to install High Sierra in a new APFS volume in the same APFS container that Catalina is using.
