In Monterona you are in the imagination of a small kid who is going to visit his grandma in Italy soon. Grandma tells him stories about her hometown, and you, as a player, try to build places and scenes from these stories. But there is no "right" way to build these scenes, no one is going to judge you in the end, so you can relax and just create however you want.
Creating the scene starts with placing houses, pavements, grass, and water. After that, you can put decorations and little citizens of the city. The exciting part about decorations is how objects adapt to their surroundings and your actions — if you take a flower in a pot from a table and put it on the grass, it will become a bush. If you make it bigger, you get a tree. This mechanic encourages players to experiment and brings them joy when they get some unexpected variation of an object.
Once you're done with building a scene, you are prompted to take a couple of photos in camera mode to fill out the storyboard. At this time, you can also check out how other players "imagined" the same story and maybe learn something new.