As for /etc/network/interfaces, when you install wpa-supplicant, a script hook is installed in:
/etc/network/if-down.d/etc/network/if-post-down.d/etc/network/if-pre-up.d/etc/network/if-up.d,
The hook is called wpasupplicant and is a symlink to /etc/wpa-supplicant/ifupdown.sh.
This hook will be invoked by ifup / ifdown, and the script the symlink points too tests wether the interface is wireless or not. If that is the case, the command will be passed on to wpa-supplicant.