How do I tell the Generator what to take from each reference?

Connect several images to the Generator: the first is the base (the space being transformed) and the rest are references. You give each reference a type, on its own node or in the Generator's ⚙ panel:

Image: a general reference; it takes materials, palette, light and mood, never the composition.
Material: applies that texture, color and finish to the surfaces.
Furniture: places that piece in the scene.
Element: adds that decorative object.
Scene: takes the atmosphere, the light and the mood.
Style: takes materials, palette, light and photographic treatment, without copying the building or the framing.
Winning render: an already approved render of the same project, seen from another angle, to keep materials and light consistent across views.

The base isn't chosen: it's the first image connected. To change it, reconnect the images in another order. Each model takes a maximum number of images, base included: see “Editing models”.