Hi Ricardo! Here's a quick reference guide about "in" and "at" (as well as "on"): https://www.espressoenglish.net/when-to-use-in-on-and-at/ Basically, "in" is used when we're being very specific about the location, whereas "at" is used when we're simply referring to the location in general.

By the same token, British people don't study 'in university'. 'In' is the preposition that Americans use with 'college' so there is sometimes confusion about the overlap.

In cases you're curious, 'at university' is BrE. Americans don't study 'at university.'