Yes, for the most part, you are assigned a roommate who will be a stranger to you.
However, many universities have a roommate selector section of their website where you can enter in roomie features you couldn't possibly put up with - you like to be in bed asleep at 9pm so you don't want a roommate who is up all night and sleeps to noon or beyond, you get stabby if someone snores, you are a neat freak so no 'relaxed about mess' people, please.
Universities don't guarantee they will find you the perfect roommate, but you might find you get along fairly well. If problems arise, you bring them to the RA (resident advisor) who tries to mediate. If the situation is really bad, you can ask to be moved. Realistically, the only time you might be able to move is at the start of a semester when others who hate their roommates are all trying to find somewhere else too. You might find you've left a bad situation for a worse one.
Only my DS had a truly awful experience with a roommate - there were stolen books, a complete lack of personal hygiene, and lots of other issues. The other four got along fine with theirs. They were able to contact their roommates before meeting them on drop-off day and coordinate who would bring or rent a mini fridge, microwave, electric kettle, rug for the floor/ what colour, etc. DS never bothered trying to be moved, but he probably could have been accommodated if he had tried. My DCs were used to shared bathrooms and bedrooms as there are five of them, and we had a small house. They always requested roommates from large families if possible.
They encountered and heard about a good many students who had grown up in extremely salubrious surroundings, often with live-in housekeepers, kids of 18 who had never done their own laundry, or taken public transport..
And then there are sorority and fraternity rituals and houses, truly a different world.
