How it is a made? Technique placed hard surface on model.

I found artist (Sergey Tyapkin) doing some modular assets on his stream. And i try analyze this but im not sure what happens here. He creates trim sheets, unwrap single planar (cut this planar apart) and place pieces of this planar on where he wants to be? And next export this whole meshes into  Unreal and left this floater above the meshes, and group these meshes as if there were one part, and left this in this form? 

https://www.youtube.com/watch?v=Dwash3O8x9o