Nothing could be easier to remember and understand -- I don't understand why Windows hasn't adopted it yet.

Command is generally used for command execution. But also it's used as a modification for mouse clicks. So, when you click with Command button active, you can sometimes get a different outcome.

Shift key normally modifies letters to uppercase. But it also can be used as a modifier for commands. For example, Command-P displays the Print dialog, and Shift-Command-P will display Page Setup dialog.

Option is also used as a modifier to Command. But not always; you can use Option as a modifier to anything. As Human Interface Guidelines writes, it can be used as a modifier to a function that is a convenience or power user function (wait, what's the nature of the "Page Setup" function?). And it will sometimes change the contents of the menus when using the mouse or touchpad (so, for each app you can actually have two menu sets, not just one). Also it's a standard modifier for using special national letters in some locales, like ż or ó. Also it can act as a modifier for mouse clicks.

Control is also used as a modifier for Command, but also sometimes it's a standalone modifier for keys. But sometimes Control left click is used as mouse right click. But, Apple Human Interface Guidelines tell you to completely avoid the Control key when you create an app, because it's somehow used too much.

As much as possible, avoid using the Control key as a modifier. The Control key is already used extensively throughout the system. To avoid possible conflicts, use it only when necessary.

I bet this line exists mostly in order to differentiate macOS from Windows, because Control key is a common key in Windows!

Next, the Fn key is used as a key modifier (because Control, Option and Shift are apparently not enough). But also some options are launched by using the Fn key alone, without Command (e.g. fn fn will sometimes start Dictation mode). Plus, Fn+ESC will enable the Alt mode, which acts like the Alt key on a Windows keyboard.

I'll even skip the fact that the Command key uses an ancient Nordic symbol of protection, that was carved on stones and houses in order to protect the objects and their owners from evil spirits and bad luck. And in current times, it's used as a "place of cultural interest" sign, which is obviously practically the same as a menu option, right?

And what's the symbol Option uses? I don't even know.

Why the Control symbol is the same as Shift+6 symbol (both look like ^)? Why fn doesn't have any symbol, despite the fact that it's a pretty heavily used button (it's used for page down/page up)? Why Shift uses the 'up arrow' symbol, and Control also uses kind of a more minimalistic up arrow?

Some questions are better left unanswered.