Visualize a Building in a 3D City and importing into OSM

I have to create a city in 3d, Target city is not has a 3d buildings. I have 3d models for each building. a
There is a website; like this. On this website, I enter the coordinates after uploading, and voila my 3d model is included in the coordinate I entered.

So I need to make this project I have no idea which I follow the roadmap, I can speak JavaScript and C#, So there is any suggestion for me ?

Do you have 3D models of really existing buildings?

If yes, can you give us an example?

What is the source of your 3D models?

Have you verified whether those buildings are already available in the OSM database?