honestly nearly everyone starts uni knowing no one so especially at first but it does continue - people are very open to making friends, I've had people invite me to do something based on saying 'hi, what course do you do' at the door of the building, at freshers events you can just say hi and start chatting to someone, there's also loads of societies to meet people at, and if you have a lecture you can just be like 'anyone staying on campus for lunch' and see who wants to hang out
also different to school - people tend to have a few friendship groups and will spend time with different ones (course friends, flat friends, society friends) so you have lots of different opportunities to make friends