To me (an American), "what to study in college" sounds acceptable. Meanwhile, "what to study in university" sounds wrong. This suggests that these words have different grammatical attributes.
This is shown somewhat in the example sentences on m-w.com:
I applied to several public universities.
He lives near the university.
She teaches art at a local college.
He graduated from one of the country's best colleges.
She attended a business college.
He attended college for several years, but didn't graduate.
She dropped out of college.
I went to Mount Holyoke College.
When I was a junior in college, I spent a semester in Spain.
the Edinburgh College of Art
the London College of Fashion
She is attending fashion college.
Replacing the non-proper noun 'college' examples with 'university' doesn't sound right in many cases.
He attended university for several years, but didn't graduate.
She dropped out of university.
When I was a junior in university, I spent a semester in Spain.
Placing an article in front of 'university' does make it sound better.
He attended a university for several years, but didn't graduate.
She dropped out of the university.
Meaning aside, what is the difference between 'college' and 'university' that suggest a different sentence structure to make it sound "better".
While, Difference between "college" and "university" looks at the difference between the meanings, the question doesn't ask nor do the answers address the perceived grammatical difference.