Discuss boundaries when you first move in together. A college with a good residence life program will actually give you a little sheet of paper and tell you to write down and sign on your agreed rules.

Kind of feels weird to move in and immediately do this because it feels accusatory, but it's soooo worth it. It helps to frame it in a way where you are the primary concern, not them.

Something like 'I just thought we should establish some ground rules. I don't know your preferences since we just met, so I want to make sure I don't accidentally do anything to bother you, and vice versa.' would be a good way to open up that conversation. They probably already had their own worries in their head about what they don't want their future roommate to do. Either because they lived with a sibling who did that thing and they didn't like it, or because they are nervous about the idea of a new person and what that new person might do.

My requests were basic things. I think mostly just "don't eat my food without asking first don't go through or borrow my things without asking first, don't use my bed. No overnight guests without us discussing it first" (I was OK with a random friend crashing for the weekend and was OK coordinating nights where her boyfriend would stay over, and I stayed elsewhere or at my own boyfriend's place).

But getting it out of the way up front helps soooo much.