At is the preposition that is used with 'university'. I study Chemistry at university (general). I study Chemistry at the university (specific but unnamed university.) I study Chemistry at the University of Oxford (specific named university) What confuses matters is that in US English they normally say 'in college'.

It is proper to say "I study at the university," just as if you were talking about a job ("I work at a bank"). The preposition "in" would be used when you're referring to "school" or "college," which is why it can be confusing to some.

'what do you study at university '. this form is correct . but when you use in that is mean , what did you studied at university