I looked in the character window (shift-cmd-space) on my Mac and the emoji menu on iOS. Can’t find it but I see people inserting it in posts here.
Have it set with Text Expander. I also have one for HYPER which gives me these on the fly ⌃⌥⇧⌘
Initially I set up Keyboard Maestro shortcuts for each special character made by copying and pasting existing instances of the character into each KN macro. Now that I have the Alfred app, I get ⌘ and other characters out of Alfred’s “Mac symbols” snippet set.
The command key and other symbols are found in the Technical Symbols font. Available in Font Book and the Character Viewer utilities.
I also use the Mac Keys Alfred Snippet pack.
Just for general edification, the “looped square” ⌘ symbol in the Unicode world is called the “place of interest sign”, among other things, and has a deep background in Nordic symbology. The symbol was chosen by Susan Kare, who was involved in icon design for the early Macs.