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.
Key sequences can mix function keys and characters.
You can even mix mouse events with keyboard events, but we recommend against it, because such sequences are inconvenient to type in.