Mac storage basics · Disks, APFS and formats
What's the difference between an APFS container and a volume?
Short answer: A container is a section of a disk; volumes inside it are what Finder shows, and they share the container's free space instead of having fixed sizes.
An APFS container is a chunk of a disk; the volumes inside it are the drives you see in Finder. Unlike old partitions, volumes in the same container share free space — each grows as needed without being resized — so adding a volume costs no fixed space.
Your startup disk is one container holding several volumes (Macintosh HD, Macintosh HD – Data and hidden system volumes). You can add your own volume in Disk Utility with the + button, for example to separate work from personal files on an external drive.