mostly I just place some lights and then I test different positions till I think that I have found the perfect place for it In other cases I also take a look at pictures and the real-life
study the world! e.g when you want to create a forest scene, go to a real forest and take a look at everything and think about it (why does it look like that?, why is the grass under a tree lower?,…)
mostly I try to find a good combination between the size of a room and the things that I want to add. But depending on how and where you place the assets, you can let the room feel “full” or “empty” e.g a table in the middle of the room = feels more full; a table in one of the corners = feels empty
In other cases I also take a look at pictures and the real-life