So Kiara in her ongoing stream confirmed/reminded people of how these 3D models work. And it's basically what everyone suspected.
They have a variety of 3D bodies, and just plop their heads on them.
She went on to say that sometimes this clashes with the artists way of designing and drawing bodies, putting her "butt buff" for the Holosummer outfit as an example.
It would explain how Holoh3ro suddenly have Default, holosummer and Yukata 3D outfits.
I guess that maybe the Idol outfits would be an exception to this.