During the application process you will indicate your preferred supervisor and be asked to describe your research interest but both can be subject to change until you are accepted and enrolled in the program.
During your faculty interview and during on-boarding to the program (if accepted) you will have the opportunity to discuss and receive advising on both your supervisor and your research topic.
Your final research area(s) will be determined in close consultation with and approval by your supervisor, this generally occurs during the coursework stage of your program.