The prepositions in and at can have many different functions. Sometimes there is no logical reason related to why we use one preposition and not another. Practice and experience will help you.
Generally speaking at is used to describe an exact location or time.
I will meet him at 2pm. ** ** = the time stated is very exact or specific**.**
E.g. I will meet you at the library. = the exact place you will meet.
You can also use at to describe where two lines cross or intersect such as two roads that cross or a corner.
E.g. I will meet you at highway 1. and Corfield St. = where these two roads cross each other.
E.g. Meet me at the corner of Corfied road and Murray St.= where these two streets connect or join at a corner.
We can also use in to talk about time and place.
E.g. I will see him in January.
E.g. I will see him in the new year .(or in 2018).
E.g. Put the piano in the corner. = used to show a specific location within a house.
E.g. Please meet me in the library. = in refers to inside the library and at generally refers to meeting outside at the entrance (although English speakers can use both to mean the inside).
As you can see, some of these functions are logical while others have to be committed to memory. These are only a few uses and examples.
Also remember that some prepositions form part of a phrasal verb structure and don't function on their own and this confuses learners.
E.g. Please drop in when you have time. drop in = please visit