Directories are objects too

To store a directory, Git writes out an object describing the directory's contents:

         100644 blob 3a6f56f8a15f5f96411c267a7428fcfeff6f9bcd    foo
         040000 tree e79566ab21a03dfac3fb4b479b24e3d9ff379d29    doc
         100644 blob 22aa6ff4684a3080367c7bb5ec366fb3bd0364c6    bar

This object is called a tree

Each item in the tree has a mode, a type, a hash, and a name

The tree object is stored in the repo under its hash, just like a file