At tells us that the following noun is located at a specific point or location. It shows an exact position. "She's waiting at the entrance." "He's sitting on his chair at his desk." "I work at a bank." In an Enclosed Space In tells us the noun is in an enclosed space (surround or closed off on all sides). Basically, when something is inside something. In a box. In a room. In a country. Example: She's at the library. She's in the library. She's at the library. - the emphasis is on the her location and the type of place she has gone to. She's in the library. - the emphasis is on the type of building she is in.

We usually use 'in' for an enclosed space and' at' for a point - In a taxi At reception