What is Their Main Difference?
The main difference between prepositions 'in' or 'at' is in what they specify. 'In' is less specific times and location while 'at' is for more specific times and location.
Talking about Location
We use 'in' and 'at' to show location as they are prepositions of place. 'In' is used with non-specific locations like name of countries, cities. 'At' is used with specific locations such as addresses. Have a look:
She has been living
Here, Zurich is specific but the sentence does not specify where exactly in Zurich does the action occur.
He is currently
Here, the location of the subject is specified.
Talking about Time
'In' and 'at' are prepositions of time and therefore can be used to talk about dates and time. We use 'in' to talk about non-specific times such as 'morning,' 'evening,' 'afternoon.' 'At' is an opposite case. It is used with specific times like 'night,' 'day,' 'midnight,' and even an hour. Take a look at the following examples:
I always read
He will meet us
Are They Interchangeable?
The answer is no. We cannot replace 'in' and at' and expect the same meaning and even a grammatically correct sentence. You must be very careful with how and when you use them.