Which One Should Be Used?
Both 'in university' and 'at university' are correct and can be used. 'In university' is used to refer to the university as an institution. 'At university' is used to refer to the physical presence at the building or campus.
'In university' is used for two purposes. It can be used to refer to the university as an institution. This means that the subject can be a student, an instructor, or a worker of the university. It can also be used to talk about the field of study. Have a look:
I study English Literature
He has been teaching
'At university' is used to refer to the location of the university. This can be the building and the campus of the university. Note that the subject may or may not be a student or a staff member. Take a look at the following examples:
I will meet you
He is a nerd, he spends most of his time