They are called custom manufacturers. Each will make custom house packages for individual homebuyers, including enough houses to stock a new development for a builder. There are also catalog manufacturers, who turn out a line of houses shown in their catalogs. You may buy any one, often with optional changes, if desired. Many makers will also modify their standard plans on request, though generally this costs extra.
Some manufacturers do both.
They make their own line of houses shown in their catalogs and on request produce custom house packages for buyers. Many banks and other mortgage lenders readily approve a mortgage loan on a factory house because the house has been evaluated and financed before. The factory house generally comes with better credentials, including certificates of approval by state, regional and national building codes. The house construction is clearly acceptable. A buyer might also be blessed with favorable terms, and the mortgage application ordinarily sails through processing.