How can I conduct user research on a product that already exists?

I am working on a school project as my team's UX researcher. Our product has already been designed, prototyped, and launched, without having conducted any research.

The team wants me to create questions on the already existing features and concepts. Isn't this backwards? How can I conduct user research after the product already exists? I have never really worked on a project where UX research was not conducted first, in order to find the users' needs and desires. How should I go about this?