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`, that invokes in turn `/etc/wpa-supplicant/functions.sh`.

This hook will be invoked by `runparts` in `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`.

For acting on the commands, passing them to `/sbin/wpa-supplicant`, it seems the script `/sbin/wpa_action` and the binary `/sbin/wpa_cli` are used.

I found an old page talking about this here:

http://manual.siduction.org/inet-wpa

> As mentioned earlier, each wpa_supplicant specific element is prefixed
> with 'wpa-'. Each element correlates to a property of wpa_supplicant
> described in the wpa_supplicant.conf(5), wpa_supplicant(8) and
> wpa_cli(8) manpages.
> 
> The supplicant is launched without any pre-configuration whatsoever,
> and wpa_cli forms a network configuration from the input provided by
> the 'wpa-*' lines. Initially, wpa_supplicant/wpa_cli does not directly
> set the properties of the device (like setting an essid with iwconfig,
> for example), rather it informs the device of what access point is
> suitable to associate with. Once the device has scanned the area, and
> found that the suitable access point is available for use, these
> properties are set.
> 
> The script that does all the work is located at:
> 
> /etc/wpa_supplicant/ifupdown.sh /etc/wpa_supplicant/functions.sh
> ifupdown.sh is executed by run-parts, which in turn is invoked by
> ifupdown during the 'pre-up', 'pre-down' and 'post-down' phases.
> 
> In the 'pre-up' phase, a wpa_supplicant daemon is launched followed by
> a series of wpa_cli commands that set up a network configuration
> according to what 'wpa-' options were used in /etc/network/interfaces
> for the physical device.
> 
> If wpa-roam is used, a wpa_cli daemon is lauched in the 'post-up'
> phase.
> 
> In the 'pre-down' phase, the wpa_cli daemon is killed if it exists.
> 
> In the 'post-down' phase, the wpa_supplicant daemon is killed.