I've been using GitHub's code review features a bit recently. Something I really like is the ability for a reviewer to quickly make suggested code changes that the reviewee can can approve with a button push, instantly committing them. This saves a lot of time, especially for small, relatively insignificant changes. Overall though I find GitHub's code review a bit fiddly and awkward, and it's features aren't "easily…
To your first point - how? I didn't know you could do this.
First you hit "Start a review" when viewing a pull request. You can then click on any change and it opens a comment box - in that box, you add hit the button with a plus and minus symbol on it. It then adds the existing code in the box, and you can change it.
Better explained here: https://help.github.com/en/articles/commenting-on-a-pull-req...