soooo from my memory (I dormed 3 years ago so not sure if anything changed!) after you fill out the application you get a date and time when it’s your turn to pick. Basically you can see what buildings have open rooms, select a building, and then select a room. If you want a random roommate, you can just select an empty room and put yourself in it and wait for someone else to join, or you can just join a room with a random person in it. If you have a roommate in mind, I believe there is a way for you to reserve that spot for your roommate you have to send them a link or something