The radio firmware controls basic low-level functions like network connectivity, Wi-Fi, and GPS. For several years, though, "radio" means "baseband", and controls network connectivity aspects only, while WiFi and GPS are governed by OS drivers, like any other devices.
You interact with a program through its interface.
commonly refereed to as "user interface" graphical interfaces such as a web browser (i.e. inter explorer) are known as "gui's", graphic user interfaces