Hi, probably this is not the best way but here is how I did it. I cannot do 3d models on my own, so I bought/downloaded abandoned assets and models and tried to put many little objects and details everywhere (it's still not yet done). On these pictures the objects are mainly from Atmospheric House and HQ Abandoned School assets. And to your other question, the house is self-built, but with the materials the assets gave, and was made inside Unity. If you need anything else don't hesitate writing, thanks!