internal/ has most of the code that is called from main.go. This is named "internal", because that is a special golang directory name that means the folder content is only accessible within this application and not through the go package management system.
A small section for each module will be explained below.
A public function for each of ttrack's commands is available here. There is also a public "state" struct definition available, which is the input to all functions.
Currently, there is only a function that helps figure out where the config directory should be located. This function is used by main.go.
Functions for interfacing with the boltdb database are here.
Functions for interfacing with the the .tt data files are here.
Standard types used throughout ttrack can be found here. This includes "dates", "seconds" (a subset of golang's duration), and groups (a subset of strings that enforces correct group names).
This file has most of the cobra/cli specific logic. The purpose of this main file is to translate from cli arguments to internal/cmds module function calls.