Yeah so people will be a bit loosey-goosey with the terminology. But all of these changes are immutable, that is, when you change a change, you’re generating a new commit. “A VCS on top of a VCS” isn’t a terrible way of thinking about it: imagine if every commit in your repo had its own history.

Merging is making a change that has more than one parent. You can then resolve any conflicts within that change.

Splitting a change into two will update one commit with one half and make a new change with the other half.