The standard Emacs editing characters such as C-a also get their standard meanings from the global keymap. See Rebinding . When you type the character M-a in a key sequence, Emacs replaces it with ESC a . A meta key comes in as a single input event, but becomes two events for purposes of key bindings. The reason for this is historical, and we might change it someday.
Most modern keyboards have function keys as well as character keys.
Function keys send input events just as character keys do, and keymaps can have bindings for them. Thus, for most purposes, you can pretend the function keys reach Emacs directly and ignore their encoding as character sequences. Mouse buttons also produce input events. A keymap records definitions for single events. Interpreting a key sequence of multiple events involves a chain of keymaps.