@Anna Vickroy
Prefabs came 3 ways that I know of
1) Whole house prebuilt. Transported as a whole unit and using a crane to lift onto the foundation. These can be all prefinished and near move in ready with paint and appliances already completed.
2) Modular. The house is built of "boxes". A kitchen section, living room section, bedroom section and so forth. Each module is crane lifted into place. Each module can also be preassembled with appliances and finishes.
3) Panels. The panels are basically wall sections. These are carried onto the foundation and bolted/secured piece by piece. This is a good option if site access is an issue.
With the first and second option you need access for the large trucks to get near the foundation and clearance for a crane. I've heard they can lift the house over an existing two-story home in some situations that allow for it.
The beauty with prefab too is that the ADU can be constructed while the foundation and other steps can be completed simultaneously thereby speeding up the process significantly. Obviously time saves $$$.
In my situation I'll probably have to go panel construction. I've got a big city planted tree in the way (you know how the city is very strict about the trees. Can't remove unless it's diseased and even then it requires a permit and public notice to remove)