What I've seen content producers do typically is review all of the major blockbusters that get marketed to the end of the earth and back again. And they'll send use semiweekly polls within their communities to gauge which types of movies their viewers are most interested in seeing reviewed. Sometimes, they'll open up a question to their community and just try to work through all recommendations that they receive.
These people are independent content producers, though. I have to imagine that critics in more professional, corporate settings have some kind of creative lead who instructs them on what to cover. That creative lead may get their intel from a market research department - this is all speculation.
That's all very vague, but I tried! Best of luck.