The folder src/apps/ is the place where all the user-facing features are implemented.
Each app has a boot() function in the module's __init__ file. This functions assigns what function should be called if some specific message was received. In other words, it is a link between the MicroPython functions and the Protobuf messages.
This binds the message GetAddress to function get_address inside the apps.bitcoin module.
from trezor import wire
from trezor.messages import MessageType
wire.add(MessageType.GetAddress, apps.bitcoin, "get_address")