In is often used to indicate a large space or scope, such as a country, a city, a room, etc. For example, IliveinBeijing. At is often used to indicate a specific place or location, such as street number and building name. For example, "WeworkatNo. 1Hospital" (we work in the First Hospital).
Different states: in is often used to indicate being in a certain state or situation. For example, "sheisinggoodweight" (sheisinggoodweight). At is often used to indicate that you are in a certain mood or feeling. For example, "Sheisatpeacewithherself" (she has peace of mind)
Time is different: in is often used to indicate a period of time, such as year, month, season, etc. For example, "Shewasbornin 1990" (she was born in 1990). At is often used to indicate specific time points, such as hours, minutes, etc. For example, "thetrainwill8 leaves at 8 o'clock" (the train leaves at 8 o'clock).