I would say is both.

Developer's end will be the planning of a township/development. This including how they planned and designed the roads, the layout of the houses, the facilities, the landscaping etc

And also of course aftersale service, how fast they resolve defects issue etc.

Constructors' brand aka contractors will be the quality of the building, defects etc

Just that, the heavy responsibilities in this context fall under developers simply because the whole project belongs to them and is their initiatives. They are responsible to hire the right contractors to build the houses and also to ensure the QC and QA is properly done so by the contractors.