“in university”和“in the university”哪个正确?怎么用？

都正确，at
university读大学，在学校期间。
in
the
university在大学里。具体的大学，指定了固定的对象。

扩展阅读：at a factory ... vanderbilt university ... firstly ... real world camp ... on a plane ... tiangong university ... have a severe impact on ... in the united states ... in a gym ...