See if your Mac shares space across APFS volumes

If you see more than one disk in the Storage area of General settings, your internal disk has multiple volumes.

A single APFS partition (container) with two volumes.

macOS 10.14 or later uses a file system called Apple File System (APFS), which allows space to be shared between volumes on a disk. If a single APFS partition (or container) has multiple volumes, the container’s free space is shared and can be allocated to any of the volumes as needed. Each volume uses only part of the overall container, so the available space (shown in gray in the illustration above) is the total size of the container, minus the size of all volumes in the container. This overall amount of used space, including other volumes in the container, is indicated by a crosshatched area.

To learn more about partitions, see Partition a physical disk in Disk Utility.

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macOS User Guide

See if your Mac shares space across APFS volumes