bizGyver™
Petit Tech for macOS
macOS can set multiple APFS volumes (including bootable APFS volume groups) in one APFS container, which container is equal to partition.
When you set multiple APFS volumes to use both new and old macOS, you can continue to use a stable version of the former macOS, and can also try to use/check a new version of macOS.
Operating system can effectively use space of the storage like internal SSD on which macOS is installed by setting multiple APFS volumes in one partition as a whole storage.
For further details to backup APFS volume (group) as a whole, refer to the page "Backup APFS Volume Group" on this website.
A Mac can boot from only one macOS at once, and cannot boot with more than one macOS like emulation.
In order to use another version of macOS, you need to reboot a Mac to choose a boot volume on the startup manager.
When you reboot a Mac to start up it in "macOS Recovery" with multiple versions of macOS being installed, your Mac boot from macOS Recovery of macOS set as the startup disk.
macOS can set multiple APFS volumes (including bootable APFS volume groups) in one APFS container, which container is equal to partition.
When you set multiple APFS volumes to use both new and old macOS, you can continue to use a stable version of the former macOS, and can also try to use/check a new version of macOS.
Operating system can effectively use space of the storage like internal SSD on which macOS is installed by setting multiple APFS volumes in one partition as a whole storage.
For further details to backup APFS volume (group) as a whole, refer to the page "Backup APFS Volume Group" on this website.
A Mac can boot from only one macOS at once, and cannot boot with more than one macOS like emulation.
In order to use another version of macOS, you need to reboot a Mac to choose a boot volume on the startup manager.
When you reboot a Mac to start up it in "macOS Recovery" with multiple versions of macOS being installed, your Mac boot from macOS Recovery of macOS set as the startup disk.