A YDR file is GTA V's format for drawable models. It stores the actual 3D mesh, materials and skinning data for one model, whether that model is a building, a piece of a vehicle, a prop or just some geometry. Glossary anchor.

When a YMAP tells you to 'put this object here', the YDR is the object.

What YDRs hold

Vertex and index data. Material assignments. Skeleton information (for animated objects). Texture references. LOD meshes for distance rendering. Everything required to render a model.

Custom 3D content for GTA, whether it be new buildings or new MLO interiors, eventually becomes a YDR through export plugins in 3ds Max or Blender.