Git commits only changed lines of a file, or the entire file?

Question

git

Let's say I had a file already added to the git repository.
In this file, I change one line.
When I push a modified file, does git push only the changed line, or the entire file?
I would like to receive an answer with a link to the documentation.

Answer 1

see here . explains how git reads and saves changes

Answer 2

A commit sends nothing at all
git push will call git-send-pack at some point
https://git-scm.com/docs/git-send-pack