The Wi-Fi switcher is not really an "app" that you can launch.

It will appear in the status menu. That is the menu that appears when you tap on the status bar - the little place with the current time and little icons.

To answer the question in the topic title: apps are (or should be) installed to the /opt folder on the device. Note that it is unreachable from the default file manager as users shouldn't really mess around with it.
Although you can go there from a root terminal.