Which types of structural framing systems and materials are most commonly used in the US for residences? For office buildings?

US Residences: Concrete Basement Walls and Timber Frame for upper levels.

US Office Buildings: Concrete

Why do different teams of designers and subcontractors link and share their models during the design process? 

So the models can be shared without changing the other contractors original design.

How do you think design coordination was done before we started using digital models? 

Overlapping blueprints first. Then on CAD comparing layouts.

What strategies can design teams use to find and avoid clashes prior to the start of construction? 

Glue models on a coordination space.

At what point in the project development process should design coordination start? 

Present along all the life cycle of the project.