There's nothing unrelated about git checkout, just your understanding of what it does. It "updates files in the working tree" to what is in the specified ref, o

There's nothing unrelated about git checkout, just your understanding of what it does. It "updates files in the working tree" to what is in the specified ref, or if you don't specify the ref, to what is in the index. The fact that you think of those as two separate operations is because you are used to systems that treat them as such.