reference: https://git-scm.com/book/en/v2/Git-Internals-Plumbing-and-Porcelain

• “Git is fundamentally a content-addressable filesystem with a VCS user interface written on top of it”
• plumbing commands → subcommands that do low-level work and are designed to be chained together UNIX-style or called from scripts
• porcelain commands → user-friendly commands

.git directory structure

• git init
  • creates .git directory
    • everything that Git stores and manipulates located here
    • can be used to back up or clone repository

config
description
HEAD
hooks/
info/
objects/
refs/

• description — used by GitWeb
• config — project-specific configuration options
• info — keeps a global exclude file for ignored patterns not tracked in .gitignore
• hooks — client- or server-side hook scripts (more https://git-scm.com/book/en/v2/ch00/_git_hooks)
• HEAD — points to the branch currently checked out
• index — Git stores staging information
• objects — stores all the content for your database
• refs — stores pointers into commit objects in that data (branches, tags, remotes, and more)

objects

• content-addressable filesystem ⇒ key-value data store where inserting any kind of content into a Git repository and Git will produce a unique key to be used to retrieve that content
• git hash-object → takes some data, stores it into .git/objects and returns a unique key for that object
  • takes the content provided and returns a unique key that would be used to store it into the Git database
• contains pack/ and info/ subdirectories