When a marked paper is open for review, Testy moves into the page's own bottom bar as a pill reading Ask Testy, next to Next. This is the most useful place he sits, because he can see exactly what is on the screen.
What he can see
Testy is given the question currently on screen, the options, what the student chose, the correct answer and the explanation that goes with it. That means he can explain the idea in the student's own language rather than repeating the written explanation, and then offer a similar question to try.
The two suggestions above the message box on that page are Explain this question and Why was I wrong?.
What he will not do
Testy is only given the answers when the paper is finished and marked. On a paper that is unfinished or still being marked he is told there are no answers available, because an unfinished attempt can be gone back into. He also gets nothing at all on a paper that belongs to someone else.
Practising what went wrong
From a marked question Testy can send the student straight into the matching LearningHub skill, and the skill list marks which one it was. Questions he sets in the conversation count against Testy's allowance, not the LearningHub one.
Which allowance this uses
Reviewing a marked paper is tutoring, so Testy on this page spends the tutoring allowance, the same one as his own page, not the smaller allowance he uses elsewhere. See How much Testy you get.
The page's own explanation
The review page also has its own written Explanation panel for each question. Opening Testy closes that panel and opening the panel closes Testy, so they never sit on top of each other.