Git

The de-facto version control system.

Submodules

one way to vendor dependencies or use another project within the worktree
Git will inspect .gitmodules as the submodule configuration file; while it is plain-text, it is mostly managed with the git submodule subcommand
to clone an existing project that uses submodules, you can add the --recursive flag on the git fetch subcommand; if it's already clone, you can proceed as normal (e.g., git submodule init && git submodule update)