New volume that is used only for system files. “Volume Group” two separate volumes, but presented to OS as one.
Firmlink, similar to symlink. But support backwards and forwards path traversal. Only for directories, from system volume to data volume. Created by installer and then not changed.
You can remount the drive read-write by disabling SIP, until reboot.
iOS
Access files on network shares and USB storage. From files app.