The key is in the name: "module".
A modular house is a house built, section by section, in a factory setting. These are massive climate-controlled installations that assemble homes in accordance with the International Residential Code (IRC), which requires compliance with all state and local building regulations.
IRC is similar to the US Department of Housing and Urban Development (HUD) in that they both offer quality and safety standards. When it comes to manufactured homes, HUD requires each home to have a steel frame attached for ease of transportation, while homes built to IRC specifications are set on a permanent foundation - just like a home built in place.
When a modular house has been fully fabricated, the sections are then transported to the job site (usually via a semi-truck) and, using cranes, are assembled on a poured foundation.