Many Emacs functions do not have a key-command. Emacs has thousands of commands and there are only so many keys you can remember. All emacs commands, even those already described, can be run by entering M-x. M-x will respond with command and let you type in the command name.
In emacs commands and mode-line questions, you only type as much as you need, then press space to complete the command. Emacs will wait for you to press Enter to make certain this is the correct command. For example, you can enter M-x d e s c space and emacs will fill the line to say describe-; you can then type m o space and emacs will fill out the rest of the line to say "describe-mode".