is mac hd a volume or a disk/drive???

With the advent of APFS some years ago, the logical structure, and therefore naming conventions, changed.

Under APFS, the media (HDD or SSD) is considered to be a DEVICE.

• When you format an SSD or HDD under APFS you get what is called a CONTAINER (aka Container disk"n"). You used to know this entity as a PARTITION but now it's called an APFS Container and it has different properties.
• When you install macOS on an APFS formatted device, in the APFS Container you get a number of VOLUMES, only one of which is visible in the Finder; the others are hidden.
• The visible Volume is by default called "Macintosh HD" but it is actually a VOLUME GROUP, a combination of 2 Volumes: "Macintosh HD" and "Macintosh HD Data." This Volume Group is presented to the Finder as a single entity called Macintosh HD and shows up on the desktop as if it were a drive; you can even change its name & its icon image same as possible with any traditional drive.
  • The Macintosh HD volume contains the macOS system and is off limits to users.
  • The Macintosh HD Data volume is where all user-installed apps, user home folders & user data live
    • They are related to each other and share the entire available space in the Container.
    • The "HD" part of the name appears to be just a carry-over from previous naming conventions and I believe it is not intended to convey the idea that it is an actual hard drive.
• The hidden Volumes are called Preboot, Recovery, VM and Macintosh HD Snapshot
• If you create additional Volumes yourself, each will show up on your desktop with an icon that looks like a drive.

If you want more detail, I suggest reading these articles. Note that disk structure continues to evolve with each version of macOS and in particular in the evolution from Intel to Apple Silicon:

• Making sense of Disk Utility and disk structure in macOS 10.13-10.15 (2019)
• Boot volume layout (2020)
• Boot disk layout in macOS Monterey (2021)

The Finder {Preferences/Settings} items that refer to "Hard disks" really seem to be about "Internal volumes" of any sort, whether on HDD or SSD.

What you see in the Finder results from a combination of several factors:

• Most Macs only have a single logical internal storage device – Mac Pro towers with extra drives mounted on PCIe cards excepted; and
• Apple continues to use the "hard disk" terminology even though they are now using SSDs.
• Apple likes to hide things like Recovery partitions, Sealed System Volumes, and the internal structure of the now-discontinued Fusion Drives from GUI users. This stuff is visible in Disk Utility and the Unix CLI environment, and backup utilities like Carbon Copy Cloner sometimes have to deal with it – but the idea is that at GUI user level, all this stuff is hidden under the hood (like the arrangement of gears in your car's transmission).

Volume is technically accurate whereas hard disks have not been incorporated in Macs for about five years now. All Macs have volumes. If you have an older Mac, those volumes may reside on a hard disk drive but exactly where or on which medium they reside (or the particular file format) doesn't really matter to its operating system.

Does an iPhone have a disk? Does it have volumes? Do most iPhone users know the difference, or even care? Should they?

Apple does what they do and they don't discuss the reasons. Should you wish to address Apple's perceived shortcomings, Contact Apple. This site is inappropriate for that sort of discussion.

Apple's engineers probably know, but knowing the difference and caring about it don't always mesh.

Apple's marketing materials are often ahead of their support documents in that regard though. "Storage" is clearly indicated these days, and so is "memory" for that matter. Users still confuse the two. But in the end it hardly matters. Ask an iPhone user how much memory it has, and they'll probably have no idea. They certainly won't know what file format it uses, how many volumes it has, how many containers, etc, and those are no different than a Mac.

The only difference is that Mac users the ability to customize its storage... for now.