The longer the statement is, the more confused I am.
***** NOT A TEACHER *****
Hello, Odessa Dawn:
May I share a few ideas (not "answers")?
"What makes you write 'informally" is 'five pics,' isn't it?"
1. When you have time, it might be helpful if you read all that you can about the word "what."
a. Sometimes it is NOT a question word, such as "What is your name?"
b. Sometimes it is a so-called indefinite relative pronoun. That is, it = "THAT WHICH."
c. If we were to diagram your sentence, we would get a sentence something like:
That
which makes you write 'informally' is 'five pics,' isn't it?
i.The adjective clause "which makes you write 'informally' " can be forgotten ONLY for the sake of analysis. (Of course, you need it when speaking or writing.)
d. Thus we get the simpler sentence: That is 'five pics," isn't it?
*****
I have found it easier to analyze sentences if I simplify them as much as possible.
Let's look at your original sentence:
"That's the guy that / who won the pools, isn't it?"
For analysis ONLY, let's remove the adjective clause "that / who won the pools." We then get the simpler sentence:
"That's the guy, isn't it?" (By the way, that is how you correctly titled your thread!)
James