Final answer:
The primary modifier key on Apple systems analogous to the Ctrl key on Windows is the Command key, represented by the symbol ⌘, which is used for common keyboard shortcuts.
Step-by-step explanation:
The primary modifier key on Apple systems, which serves a similar function to the Ctrl key on Windows systems, is the Command key. Often represented by a symbol that resembles a looped square (⌘), this key is used in conjunction with other keys to execute a variety of commands within macOS, such as Command-C for copy and Command-V for paste. The Option key is used for more specific alternative inputs, the Shift key modifies the case of typed characters or extends the selection, and the Function key is used to access special functions on the function keys.