This is a pretty good guide, but it's marred by one major error:
"the index is your staging area where things go after using git add before they get committed."
That's not right. The index is where things go after MAKING EDITS IN THE WORKING DIRECTORY before they get committed. (What does "after using git" even mean?)
"the index is your staging area where things go after using git add before they get committed."
That's not right. The index is where things go after MAKING EDITS IN THE WORKING DIRECTORY before they get committed. (What does "after using git" even mean?)