What is the keyboard combination for the ⌘ symbol?
As the other answers mention, by default there is no easy way to enter this symbol. You could use a text substitution as well (none of the other answers so far mentioned that), but that’s another imperfect solution.

If you need this (or any other Unicode glyph) frequently and want to be able to type it on your keyboard, your best bet is to create a custom keyboard layout. That way, you can assign a keyboard shortcut of your choosing to the ⌘ symbol.

Using my custom QWERTY layout, for example, I can use ⌥+Shift+C to enter ⌘.