Depends on your relationship with your roommate.

1. For example, if we have a good relationship in our dormitory, we usually secretly buy a cake for the person whose birthday it is. Before her birthday, we would secretly create a small group to choose cake styles for her. On her birthday, we will have a dinner party during the day, and we will make arrangements for paying the bill. From the menu, each of us chooses a dish. Of course, when choosing dishes, we have to exclude dishes that our roommates are taboo on. However, my roommates and I are all snack food, so there are almost no taboos, haha. At night, I secretly took out the cake, put candles in it, turned off the lights, made a wish, and sang happy birthday to her. It moved her to pieces. Of course, she will also buy some other things as a treat, such as pizza, which is a good choice. The picture below is what my roommate treated us to when he passed away. We thought it was very good and we could choose from several different flavors. For example, durian pizza and ham pizza can be ordered as takeout. The price is not too expensive and is delicious.

2. If you have a normal relationship with your roommates and they don’t make any preparations for your birthday, then you don’t need to prepare anything for them. Cake is something that should be bought for the person whose birthday it is. You can be kind but you don’t have to be too kind. Friendships in college are often not as sincere and simple as those in middle school and high school. After all, a relationship requires the mutual maintenance of both parties. If they don't express anything to you from beginning to end, there is no need to waste your time on them. Why not save the money on the cake and buy yourself a bouquet of flowers and have fun. Your birthday is your biggest.

3. If your relationship is very delicate, you can also refer to your roommate’s previous birthday celebrations. If she bought a cake for you to eat on her birthday, you can also do the same. If there is no expression , you don’t need to do it either. However, if you really feel that it is inappropriate not to treat others to a cake on your birthday, you can also buy a 6-inch or 8-inch cake to show your appreciation. After all, it's your birthday, you can do whatever you want.

I hope my answer will be helpful to you and help you have a happy birthday!