different between at and in?

Both "at" and "in" are propositions, and both have more than one function. As a Prepositions of Place, "at" can be used to identify a mostly specific location. For example, ... As a Preposition of Place, "in" is considered less specific than "at," but that too is contextual.

AT -AT describes position at a specific point.  It's very cold at the bottom of the ocean. -AT is also used with a larger place (a store, bank, airport etc) Let's meet at the bank -AT is used before the names of group activities at a party, at a meeting, at a play, at a parade IN -IN refers to position within large areas or in spaces that surround something on all sides My keys are in my bag. He is not in his office.