In means inside something and enclosed by it: You’ll find the scissors in the top drawer.
In is used with cities and countries. We also say: in the country (side) and ”in the city”, Living in the country is cheaper than living in a large city.
In a number of fixed expressions: in the middle/ in the centre, in the queue/ in (the) line, in the sky, in the world, in the picture / in the photograph
To talk about areas of activity: He works in publishing.